A mousse can be made either from raw fish and then cooked or from cooked fish, and can be served either hot or cold. There are also many ways of making a mousse. The former, which appears most frequently in the book, is used for making quennelles, for cooking in a mould, as the basis for a fish terrine, and for filling or spreading on to other fish.
It is not easy to make a perfect mousse every time without practice, but it is not as difficult as many people seem to think as long as certain rules and procedures are followed every time. All the ingredients must be chilled beforehand and should not be allowed to warm up too much during preparation. Always use fresh fish for a mousse, never frozen – although frozen fish will do, it is much more difficult to achieve a good result. Finally, always test the mousse when you have finished preparing it and before cooking. To do this, do not use all of the cream called for in the recipe, but hold a little back, about a third. To test, drop a teaspoon of the mousse mixture into simmering water until it is cooked. If the texture is rubbery or too firm, then add more cream and test again until the desired consistency is reached. A mousse should be so light that it almost floats away, but not so light that it falls apart.
The best fish to use for a mousse are any of the firm white fish such as turbot, brill, monkfish, and Dover sole. Salmon also makes a very good mousse, as does pike. In the shellfish line, scallops and lobster make excellent mousses.