Coulis of fish or fish gravy is one and the same thing.
Boil hard the eggs, and put the yolks in a mortar. Take the pike, clean, prepare, and broil it as directed; split it open, take all the bones and skin off, put the flesh in the mortar with the yolks, and pound the whole, and knead it with a little butter. Place a little butter, of the size of a walnut, in a stewpan, and set it on a good fire; when melted, fry in it till of a golden color, the carrots and onions cut in slices; after that add also the bay-leaf, parsley, thyme, a little isinglass, the eggs and fish, and cover with water; simmer gently about one hour and a half, and strain.
If found too thin after it is strained, set it back on the fire, add a little more isinglass, and simmer fifteen minutes longer.