You will need: Two or more fresh fish frames, as large and meaty as you can find, from lean fish such as sole, flounder, cod, hake, haddock, sea bass, or halibut, and 2-1/2 to 3 quarts of water.
To make: Wash the fish frames. Cut out and discard the gills (pairs of feathery red tissues attached to either side of the throat). Whack the head and bones into pieces that will fit into a roomy kettle. Pour in the water to cover and bring to a boil. Skim off scum for a few minutes as it continues to rise; cover loosely and simmer 20 minutes. Strain. That’s all there is to a fish stock.
To store: Refrigerate uncovered; cover when cold. Chilled, the stock will keep for a day or two. It may also be frozen.