Chill mold for mousse in freezer. Blend herring, 1 at a time, to a fine puree. Strain through a fine sieve. Puree apples with Calvados, and add to herring puree. Stir in cream. Season with salt and pepper.
Dissolve gelatin in water over low heat. Do not let boil. Set aside to cool.
Fold egg whites into herring-apple puree, then whisk in cooled gelatin. Pour in chilled mold and place in refrigerator to set.
To serve, unmold and slice.
Heat 2 tablespoons butter in large pan over medium heat. Add onions and gently cook. Then add 1/2 of the white wine, half of the ketchup, and half of the raisins. Saute for a few minutes. Season with salt and pepper.
In a separate pan, heat remaining butter over medium heat. Add mushrooms and cook gently. Add the remaining wine, ketchup, and raisins. Season with salt and pepper.
Chill each mixture in separate bowls until ready to serve.
Drain brains and place in saucepan with fresh water to cover. Bring to boil, over medium heat. Remove and plunge into a bowl of ice water to stop cooking. Once cooled, drain, trim off fat and skin, and cut into 1-inch pieces. Set aside.
Blend together sugar, vinegar, oil and cayenne. Pour over brains and chill for 1 hour.