Oenogarum/hydrogarum

Oenogarum was a common component of a Roman meal and in many respects it would resemble a simple salad dressing such as vinaigrette. Pepper, and occasionally other spices, are mixed with fish sauce and oil and then sweetened with wine and sometimes one or more of the syrups described above.

Again like vinaigrette, a sauce like this is unstable and needs to be beaten frequently to maintain an emulsion which can be poured over food or used for dipping. It will separate if left to stand for any time. We find surprising evidence from wear-marks in the small Samian-ware bowls that were used at table that diners often did whisk their sauces themselves.

Other oenogarums can be thickened with starch to form a stable emulsion for pouring over a finished dish. A number of recipes in the original text have precise quantities, yet the ratio of fish sauce to wine and oil is often quite varied.  

A classic oenogarum recipe is found in Apicius Book 4.5.1 where a patina, a kind of thick frittata of meat, fish, and vegetables is turned out on to a plate and this sauce poured over it.

‘make an oenogarum for it like this: pound pepper and lovage, pound again, pour on liquamen (garum/fish sauce) and wine. Flavour with passum or with sweet wine. Blend it in a little pan; put in a little oil and bring it to heat; when it is simmering thicken it with starch’

The thickening is a wheat starch, which can be replaced with arrowroot or cornflour. The sauce is flavorful, so only a little is needed; in addition to the patina recommended in Apicius, the sauce may be an accompaniment to simple chicken. Grainger strongly recommends sourcing the Vietnamese Red Boat brand fish sauce, as it has a rounder, less aggressive flavor than many of the Thai brands.

Recipe from https://www.culinaryhistoriansny.org/:

1/4 cup Red boat fish sauce 
1/4 cup red wine 
3 tablespoons plus 1 teaspoon dessert wine – vin santo is ideal, if pricey, although a Greek samos or a marsala work nicely

2 tablespoons olive oil

Arrowroot dissolved in a bit of water, to thicken, as needed

Simply combine the ingredients in a saucepan over low heat to thicken the sauce into a thin emulsion. It makes a nice dipping sauce for poultry, shrimp, or a frittata.

Another Recipe

There is no definitive set of proportions that will make a Roman sauce to suit all conditions. However, a basic all-purpose sweet sauce might be as follows:

1 part fish sauce
1 part medium white wine
1 part passum
1 part oil
generous freshly ground pepper

Combine all the ingredients at the last minute to pour over the salad, vegetable, etc., or use as a dipping sauce (especially ientaculum). Beat before use. A starch can be added to make a more stable emulsion to pour over finished dishes and boiled eggs.

Hydrogarum

All that is known about this is that it is a cooking sauce that resembles French ‘court bouillon’. Best educated guess is that you water down some fish sauce and add a few herbs and pepper. This is used to poach foods such as eggs and other various meats.