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.