Instructions: For Mousse:
Process fish fillets, smoked white fish, and egg yolks to smooth puree in processor, working in batches as necessary and scraping down sides occasionally. Add egg whites one by one, blending well. Gradually add cream, scraping sides of bowl as needed. Transfer mousse to bowl and place bowl in a larger bowl of ice. Stir in 1/2 cup parsley, hot pepper sauce, salt and a pinch of pepper. Adjust seasonings to taste and chill well.

Lightly butter a 12 x 3 x 4-inch ceramic terrine (a standard 9 x 3-inch loaf pan can also be used). Preheat oven to 350 F. Place well chilled smoked salmon on work surface and split it down center, removing any small bones. Cut each half into rectangles about 2 inches wide and the length of the mold, squaring off edges neatly. Cut two rectangles of smoked sturgeon to the same size.

Use a rubber spatula to spread 1/4 inch layer of mousse evenly over the bottom and sides of the mold. Lay one rectangle of smoked salmon down the center. Spread a 1/2 inch thick layer of mousse over the salmon, smoothing neatly. Top with a layer of smoked sturgeon, then 1/2 inch layer of mousse; repeat until mold is filled, ending with smooth layer of mousse. Tap filled pan gently on work surface to eliminate air bubbles.

Top with buttered aluminum foil, buttered side down. Wrap a second layer of foil over top and place mold in a roasting pan.

Place the pan on the center rack of your oven and pour hot water into roasting pan to come about halfway up outside of terrine. Bake 45 to 60 minutes, or until a skewer inserted in center comes out clean (a 9 x 3 inch mold may take as long as 1 hour and 10 minutes.) Remove the mold from the water bath and cool on rack.

For Dressing:
Place egg yolks and water in bowl of mixer and beat until creamy. Add oil very slowly. When about half of the oil has been incorporated, begin alternating oil and vinegar, adding them in a thin stream. Whisk in sour cream briefly, then Champagne. Add salt and pepper and chill dressing, thinning, if necessary, with a little milk.

To garnish; separate the hard cooked egg yolks from egg whites and chop each coarsely. Use a napkin to press whites and yolks separately through a sieve.

To serve, run a knife around the edges of the terrine, then unmold by inverting it onto a serving platter. Holding a kitchen towel dipped in hot water over the mold for a few seconds, lift off the mold. Slice the terrine with a knife dipped in cold water. Cover the bottom of each chilled serving plate with dressing. Center a slice of terrine on each plate. Surround the terrine with a ring of chopped egg yolk and egg white, onion and parsley. Place 6 small spoonfuls of caviar, 2 of each kind of roe, over the garnishes around the plate.

Serve immediately.

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