1. Whip the eggs until they form peaks.
2. In a food processor, quickly reduce the trout, horseradish, salt, and pepper.
3. Pour everything into the bowl and mix with half of the whipped cream. Mix in half of the egg whites. Gently integrate into the rest of the whipped cream. Integrate, very gently, the rest of the egg whites. Pour into 4 ramequins.
4. Cover with aluminum paper and steam for 10 minutes.
5. Turn the ramequins over onto hot plates, garnish with caramelized onions and a few jets of balsamic vinegar.
This dish can also be enjoyed cold on a salad in slices, like a terrine, decorated with just as much balsamic vinegar.