Macaroni and cheese. Even the finest American restaurants through the nineteenth century served a lot of macaroni and cheese.
Allow me to explain.
The arrival of the restaurant and the restaurant experience--that is, attending a sit-down establishment whose primary focus was providing food, for the primary of eating the specific food offered there--in America is usually linked to the 1820s and 30s, with the opening of restaurants like the Union Oyster House in Boston, and the revamping of Delmonico's in New York City into the trend-setter of elite cuisine. This wasn't the beginning of "dining out," of course, and those traditions would continue.
The most important predecessors are, loosely speaking, taverns/inns (which often overlapped in function) and coffeehouses. Travelers needed a place to eat where they stayed, of course, and non-travelers wanted something to eat while they drank. "Abundant but not delicate," was how Frances Trollope delicately described American tavern menus in 1832, and Scottish general James Edward Alexander (1803-1885) agreed wholeheartedly:
[There were] Tables covered with meat, vegetables, preserved fruit, tea, coffee, and bread--both of maize and wheat--and soft hoe and waffel [sic] cakes...
No ceremony was used; each man helped himself with his own knife and fork [from the serving dish] and reached across his neighbor...bones were picked with both hands, knives were drawn through the teeth with the edge to the lips...Beefsteaks, apple tart, and fish were seen on the same plate the one moment, and had disappeared the next!
Alexander describes a basic lineup of dishes--recognizable even to an international traveler without needing menu descriptions to inform what was being eaten. For some travelers, the lack of a description was both a curse and a blessing. In the 18th century, a Connecticut teacher and diarist had described one of her own inn...meals...as:
a twisted thing like a cable, but something whiter...[which the cook laid] "on the bord, tugg’d for life to bring it into a capacity to spread.
The tradition of eating connected with hosting would actually play a role in the evolution of "dining out"--with increased and richer international travel came nicer travelers' quarters, that is to say, hotels; and with that came nicer eating rooms and eventually restaurants that catered to locals as well as travelers.
Coffeehouses also played a role in creating a public desire for dining out, in America as they had begun to do in England by the late 18th century. At first, coffeehouse culinary offerings had sought to mirror the exotic allure and reputation of coffee from the East with heavily spiced and candied foods. In England and America, "France" was as good as the East for exotic; pastries were also on offer in some places. But it turned out people really like to be social to eat as well as to drink (alcohol or caffeine). By 1800, coffeehouses had introduced tables and booths, a waitstaff, and sometimes the types of nicer food one would not necessary be used to at home paired with a nice glass of wine! John Byng, grouchy about the state of provincial coffeehouses on his travels through countrysides, dreamed of home:
A London gentleman steps into a coffeehouse, orders venison, and turtle, in the instant; and (if known) a delicious bottle of port or claret: upon a clean cloth, without form, he dines at the moment of his appetite and walks away at the moment, he is satisfied.
Delmonico's, usually considered the first American restaurant worthy of the name, actually started life as a coffeehouse serving pastries in 1827. But its proprietors presumably noticed their clientele's focus on the food, and took a risk. They bought the building next door and, over the 1830s, turned themselves into a fine-dining restaurant.
They were fighting an uphill battle. "Abundant but not delicate" remained the dominating perception of American cuisine. Charles Dickens was horrified by the rustic nature of the dishes he ate at New York's City Hotel in 1842--duck with olives, more duck, grilled mutton chops. To defend themselves, the hotel owners and cooks could point out he was emphasizing the wrong thing: he had been served thirty-eight dishes.
The preference for abundant remained even as more and more chefs strove for delicate. If we can use menus as a guide for what people were eating, a meal would consist of one or more appetizers (soups, salads, fish); an intervening course called an entree that was basically the chance for the cook to show off something fancy (visually as well as culinarily) without having to worry about actually satisfying the patrons' stomachs; the main course of roast meat or fowl accompanied by vegetable side dishes; and then two dessert courses.
American fine dining across the middle of the 19th century, spiraling ever-upward in its closing Gilded Age years, retained an Anglo-American core but took on French trappings. Yes, French cooking was already the standard aspirational cuisine--not just the word "restaurant" is French, but "chef," "a la carte", even "cuisine" comes from the French for kitchen. So those roast duck and venison main courses were smothered in French sauces: Espagnole or poulette; or in English ones: Cumberland or jellies. And, of course, wine-based sauces in abundance.
Menus also feature some stock French and English dishes, like beef a la mode (which does not involve ice cream), pork with fried apples, lamb blanquette. Mutton was extremely popular, mostly owing to French influence rather than English. Hilariously, it was apparently also "fine dining" to feature modern American barbecue basics, namely, baked beans with pork or cooked in pork lard.
And then there were the oysters and macaroni.
These were kind of the go-to menu fillers for restaurants that wanted to save some money while still offering expansive menu options. Macaroni specifically was an American staple from the 18th century; the first pasta factory in the U.S. was apparently established in 1798. This was a heritage of English rather than directly Italian influence. London cook and author William Verral's 1759 Complete System of Cookery provided two versions of macaroni and cheese, which were prepared by mixing the pasta, cream, and cheese and then baking the dish. The Americans were not to be outdone: they produced, among other things, the Ladies' New Book of Cookery with nine macaroni dishes, many being further riffs on macaroni with cheese (and cream and butter) cooked and recooked.
As the fine-dining scene developed, spearheaded by Delmonico's success and other cities' desire to be as happening as Manhattan, other dining-out options appeared on the scene. The first is actually the appearance of, as you say, ethnic eating establishments! These were founded either within an immigrant community to serve the social function of restaurants as well as the hunger-satisfying one; or as lunchtime quick-stops for wage workers (which would eventually evolve into diners). These places do seem to have appealed primarily or entirely to the specific immigrant community they were founded and run by, though.
The second was the rise of ice cream parlours! Although ice cream had been on elite restaurant menus as a final course since the 1830s, it was actually increasing attention to women's roles in public as consumers and shoppers that spurred the popularity of ice cream parlours. They were built and marketed as the ladies' alternative to saloons and taverns--places where alcohol (the man's/adult's drink) was not sold but sweet things (children's food) were.
In 1798, John Byng preached the refined, civilized glory of the London coffeehouse over the country bumpkin alehouse. In 1898, women ducked into ice cream parlours while out shopping, maybe with a kid in tow. I think it's really interesting how directly the evolution of "dining out" across the American 19th century mirrors and even reinforces social stratifications in society. But this just goes back to the coffeehouse owners of a earlier age, and the medieval feast hosts before them, who understood that food can be a social experience as much as a culinary one.
My question to the historians is, why is it that Thais, who don't have a large immigrant population, have such a large cuisine imprint?