Purée the fish in a food processor. Note: It is important to keep all ingredients and equipment cold throughout the production process. If the material becomes too warm at any time, set it in the refrigerator and proceed only when it is chilled.

Add the egg whites and process until they are well blended in.

Force the purée through a fine sieve to remove any bits of sinew, bone, or skin. This step is sometimes omitted, but it much improves the texture.

Put the fish purée in a stainless steel bowl, and set the bowl in another bowl of crushed ice, so that the mixture stays cold.

Little by little, blend in the cream with a wooden spoon. The amount of cream you can add depends on the firmness and type of fish used. The finished mousseline should be light and fluffy but still hold its shape.

Season to taste with salt, pepper, and a dash of cayenne. (A little nutmeg may also be added.)

Test the consistency and seasoning of the mixture by poaching a small quenelle in salted water. Adjust the seasoning if necessary. If the cooked quenelle is too firm, add a little more cream. If it is not firm enough, add a little more egg white.

Keep the mixture cold and covered until needed.