The proportions of ingredients for fish mousseline, a puree of fish with cream and egg whites, will vary depending on how you will use it and the type of fish selected. The egg white strengthens the mousseline, and the cream makes it light and delicate. So, if you are using the mousseline to make quenelles, make a stronger one than if you were putting the mousseline in a mold and using it for stuffing another fish.
While petrale or Dover sole is very simple to use and needs little egg white, some other sole give off a lot of water when cooked and will require more egg white: generally, the cheaper the sole, the more egg white you will have to use. Turbot and pike are the easiest to use, requiring very little egg white, and salmon is in between. You could use a combination of fish and sea scallops, or of sole and salmon cut into pieces in the sole puree, or add mushroom duxelles, truffles, pistachios, lobster meat, or shrimp. Always test a piece in salted simmering water before you finish the puree, to check for texture and seasoning.
Put the fish in a food processor and finely puree. Add the egg whites and puree again for 2 minutes until the egg whites are fully incorporated. Put the puree through a fine sieve into a bowl set over ice. Discard the debris or save it for fish stock.
Beat in half the cream and leave the puree to sit in the iced bowl for 30 minutes. Whip the remaining cream with some salt and pepper and fold into the puree. Poach a piece of the puree to check for texture and seasoning.
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