I'll answer all I can and I'll lead by saying that middle-class diners could not access restaurants in the mid-nineteenth century. At least not the restaurants we know today.

The modern restaurant was invented in mid-eighteenth century France and appeared in major US cities by the early 1800s. Restaurants were technically open to all, but as you can imagine the prices were quite high for French fare, which was considered the epitome of fine dining at the time.

Even if you, as a middle-class diner, did have the financial capital to dine at a restaurant, the lack of cultural capital would likely have barred you from partaking. Restaurants were as much about the dining experience as they were about food. Public dining was a way for Anglo elites to confirm their social affiliations. They could read the French menus. They knew how to interact with waitstaff, which utensils to use with which dishes, etc. This is what I mean by cultural capital. Elites had it because it was learned behavior. For instance, French was the language of the learned. The middle-class, on the other hand, didn't have these forms of cultural capital. Even if you had the money to dine at a restaurant, in the middle of the nineteenth century you probably wouldn't have been able to. The system was built in such a way because the primary purpose of restaurants in this era was social distinction. These were establishments by and for elites. Paul Freedman argues that these restaurants had a snobbier atmosphere than they did in France itself. Sure snobbery existed there too, but restaurants in France tended to have a more egalitarian view of their customers.

You would be eating French fare. What exactly does this mean? It wasn't exactly the same food available in France. For one, ingredient availability made it difficult to replicate recipes. Further, some chefs from France scoffed at what Americans called French food because even if the ingredients were available, they were not of the same quality. Thus, scholars like Freedman have called the food available in these restaurants 'Frenchified English.' That is to say, French sauces (a huge part of French cuisine, perhaps the most important) paired with fairly standard meats and fish. Consider for instance a dish like "Broiled Bluefish - Sauce Maitre d'Hotel." Fairly standard fish with a French sauce. A phenomenal collection of menus is available digitally through the New York Public Library if you're interested in exploring what else these restaurants served (http://menus.nypl.org/). In short, though it wasn't exactly "French" (if that is to mean duplicating what was served in France), it was close enough that the elite clientele believed it to be "authentic."

Another important question was brought up. If the middle-class wasn't eating at restaurants in the mid-nineteenth century, when did they begin doing so for pleasure?

Between the late nineteenth and early twentieth century the US saw wave after wave of immigration. There were the German and Irish followed by Russians, Romanians, Hungarians, Italians, Chinese, and later Spaniards, Poles, Syrians, and Japanese. They needed to make a living, and many of them established "ethnic" restaurants. I use ethnic to differentiate them from the dominant French ones. Immigrant purveyors of food largely catered to the urban middle class. Their prices were affordable enough. A middle-class base that could not prior eat out for pleasure took great joy in ethnic restaurants. By 1900 middle-class diners were enjoying so-called culinary cosmopolitanism, and more to your point they were eating at restaurants for pleasure. On a slight tangent this became much more popular in the post-war era when corporations built chain diners specifically targeting middle class families.

The impact of these immigrant restaurants extended beyond the middle class. The number of restaurants in the US saw a 400% rise between 1880 and 1930. Competition became so fierce that elite restaurants could no longer afford to simply cater to the wealthy. They, too, saw the benefit to catering to middle-class patrons; though non-elites did not have as much money, they had the numbers. Attracting the largest market gained importance. In turn, formerly elite restaurants printed their menus in English, reduced costs, and served a wider range of American and ethnic as opposed to just French fare. French restaurants that catered to elites remained to be sure, but very few. This certainly left its mark. If you look at the American foodscape today, how many French restaurants are there in major cities compared to Italian, Japanese, Chinese, etc.?

• Ferguson, Priscilla Parkhurst. Accounting for Taste: The Triumph of French Cuisine. Chicago, IL: University of Chicago Press, 2006.

• Freedman, Paul. "American Restaurants and Cuisine in the Mid–Nineteenth Century." The New England Quarterly 84, no. 1 (2011): 5-59.

• Freedman, Paul. Ten Restaurants that Changed America. New York: Liveright Publishing Corporation, 2016.

• Gabaccia, Donna R. We Are What We Eat: Ethnic Food and the Making of Americans. Harvard University Press, 1998.

• Haley, Andrew P. "Restaurant Culture." In The Routledge History of American Foodways, edited by Michael D. Wise and Jennifer Jensen Wallach. New York: Routledge, 2016.

• Haley, Andrew P. Turning the Tables: Restaurants and the Rise of the American Middle Class, 1880-1920. Chapel Hill: University of North Carolina Press, 2011.

• Hurley, Andrew. "From Hash House to Family Restaurant: The Transformation of the Diner and Post-World War II Consumer Culture." The Journal of American History 83, no. 4 (1997): 1282-1308.

• Trubek, Amy B. Haute Cuisine: How the French Invented the Culinary Profession. Philadelphia, PA: University of Pennsylvania Press, 2001.

Further question: at what point did the middle class begin to eat out for pleasure, opposed to necessity (as they would have done at, say, travellers inns)?

Following up on my initial question: Out of curiosity, Is there anyone knowledgeable who can provide details about other cities? London, Paris, Berlin? Or further afield - Tokyo, Beijing...? I expect the clientele will be the same as the US response (upper-class); what about other aspects of the experience?

Might I recommend "Oscar of the Waldorf" an old cookbook by the maitre d'Hotel of the Waldorf in New York. Also goes in on the actual running of the kitchen, and dishes https://archive.org/details/tchirkycookbook00tschrich