This is a surprisingly popular question in this subreddit! Both /u/gothwalk and /u/sunagainstgold have good comments answering the question What was Italian lasagna in the late 14th century made of? (that is, before they had tomatoes). Similarly, /u/tomasdecourcy goes to the trouble of detailing recipes for 15th century macaroni amongst other pasta styles.
I knew one day that B.A. would be more that just an expensive piece of paper!
If you consider Roman food to just be Italian but very very old, then go ahead and look up De Re Coquinaria. It's a collection of recipes from the 4th-5th century AD that are attributed to one Marcus Gavius Apicius. It's more than likely that the recipes in the book were attributed to him, or he was even made up like a certain Naughtius Maximus. Either way the recipes were suited to the ruling aristocracy, containing such extravagances as stuffed dates covered with honey, roasted flamingo with feathers still attached, and stuffed dormouse.
That being said, the ubiquity of the tomato in modern Italian cuisine is instead taken up by garum and liquamen, or fish sauce. Similar to contemporary Thai fish sauce, garum adds an umami, or savory kick to dishes without adding a fishy taste. Garum itself was made of fermented blood and viscera of fish such as mackerel with salt, and used as a condiment like we would use ketchup. Liquamen itself, however, was made by fermenting and brining entire fish bodies such as anchovies, and was used as an actual cooking ingrediant. There is no mention on how to make garum or liquamen in De Re Coquinaria, as it was so common that it was assumed that the chef would have it on hand already. It would be as if Martha Stewart explained how to make vinegar. One interesting tidbit about garum is that the fermenting process was so pungent that there were laws put in place banning its production within the city limits of Rome.
In regards to actual dishes, the most Roman peasantry had access to was wheat for bread. Cato says,
Recipe for kneaded bread: wash both your hands and a bowl thoroughly. Pour flour into the bowl, add water gradually, and knead well. When it is well kneaded, roll it out and bake it under an earthenware lid. -Cato, On Agriculture, 74
Which is made like sourdough bread today due to the lack of commercial yeast and readily available salt. Cabbage was also quite popular with the masses, including Cato the Elder who sang praises of it's medicinal qualities.
Cook the cabbage in soda water, then squeeze all the water out. Chop the cabbage up finely. Crush and add pepper, lovage, satury, and onions, followed by fish sauce, olive oil, and raisin wine. - Apicius, 3.15.1
However, my favorite recipe is Parthian Chicken.
Prepare and dress some chicken. Grind pepper, lovage, and caraway seeds, moisten them with fish sauce, and add wine to taste. Put the chicken into an earthenware pot, pour the seasoning over, and add some wine and laser. Braise the chicken, and when it is finished, sprinkle with some pepper and serve. - Apicius, 6.9.3
Happy Cooking!
Sources:
De Re Coquinaria - Apicius
Cooking Apicius - Sally Grainger