The best way to appreciate the flavour of Roman food is to try it… so here is an example from the famous gourmet Apicius, written during the first century A.D. and adjusted to the style of modern recipes:
Fish fillets with turnips in sauce
6 medium turnips
2 pounds fish fillets
1 cup fish stock
Oil for frying
White wine or cider vinegar
1 tablespoon olive oil or butter
1 tablespoon flour
3/4 cup fish stock
1/4 cup white wine
1 teaspoon honey
1/2 teaspoon ground cumin
A few crushed laurel berries or 1/4 teaspoon ground pepper
Pinch powedered saffron
Peel the turnips and boil or steam until soft. Drain well. Simmer the fish fillets in fish stock until half cooked, drain and set aside. For the sauce, warm the oil in a pan over a low heat; add the flour, stirring well. Gradually add stock, then wine, honey, cumin, laurel and saffron, stirring constantly. Raise the heat and bring to the boil; lower the heat and simmer for 25 minutes, stirring occasionally. Meanwhile mash the turnips and coat the fish fillets with the paste. Deep-fry in oil. When cooked, place the fillets on a serving dish, pour the sauce over them and serve with a dash of vinegar.
The only problematic ingredient in this recipe is the fish stock, or liquamen, which is ubiquious in Apicius’s book. Liquamen was generally bought by the Romans ready-made from specialist manufacturers and occurs in Apicius’s recipes in the same way that ketchup or curry powder might appear in ours. Because of this it is difficult to replicate its flavour. The nearest equivalents today are the fermented fish sauces of the Orient (Thai nam pla, Vietnamese nuoc mam, Philippino patis and Cambodian tuk trey). Although these taste more salty than fishy, they can be bought from specialist shops as a liquamen substitute. Otherwise, use a light soy sauce, the stock from steamed fish or a combination.
Or, if you are feeling adventurous, try the recipe for quick homemade liquamen given by the ancient Greek writer Athenaeus: Dissolve enough salt in water until an egg will float in it; add fish scraps and oregano and boil; allow to cool, and strain several times until clear.
Source: heavily copied from "Ancient Inventions" by Peter James and Nick Thorpe.