From: The Thorough Good Cook

Sauces: 23. Fish Gravy (Coulis de Poisson)

Put into a saucepan a piece of butter, a few slices of onion, a carrot cut in pieces; then add the heads, the remains, and the bones of any fish; let the vegetables simmer without burning, moisten with a vegetable stock, and when it boils, thicken in the same fashion as a meat gravy, and use it in the same way.