All ingredients and equipment must be cold to begin and kept cold throughout the procedure.
- Purée the fish in a food processor fitted with the metal blade or in a meat grinder fitted with the finest disk.
- To obtain the smoothest texture, push the purée through a very fine wire sieve (tamis).
- Transfer to a clean bowl and refrigerate for at least an hour or until very cold.
- Place the chilled purée over a bowl filled with ice.
- Beat in egg whites, one at a time.
- Gradually begin to add very cold heavy cream, beginning with about 125 milliliters (½ cup plus 1 teaspoon) for every pound of fish purée.
- Add seasonings.
- To check for proper consistency and seasoning, bring a small saucepan of water to boil. Shape a small quenelle of the mixture and poach it briefly in the boiling water.
- Drain well and taste. If necessary, continue adding cream until the mousseline has a very light, creamy texture. If necessary, adjust the seasonings.
- If the mousseline warms or softens at any point, return it to the refrigerator until well chilled.
- Proceed to make quenelles or use as a stuffing in a specific recipe.