White sauce for fish
from https://leslefts.blogspot.com/2014/05/beyond-apicius-2-recipes-from-other.html

Galen contrasts “white sauce” with sauces made with wine, so he may have meant by it only a sauce made without colored liquids. In this case, his instructions, presumably for this “white sauce” are to “pour a great deal of water, a little oil and with the oil leek and dill." This would basically be a very dilute leek-dill sauce.

Oribasius quotes very similar instructions for “white sauce” from Galen, adding only that one should add salt after the mixture has boiled a while. Further on, he recommends the sauce in general for fish with soft flesh and the specific instructions for this use make slightly more sense: “After having poured a great deal of water on the fish, pour sufficient oil with a little dill and leek: then one half-cooks the fish and adds the necessary salt for it to not be too salty.”

In this version then the water is largely used to cook the fish and then flavored with oil, dill and leek, and after a little salt.