1. Make the court bouillon: Combine the onion, carrot, celery, leek, fennel, thyme, dill, salt, peppercorns, and 2½ cups cold water in a saucepan. Bring to a boil over high heat. Cover and simmer for 25 minutes. Strain, discarding the solids; reserve the court bouillon. (You may make the court bouillon ahead. Cover tightly and freeze until ready to prepare the rillettes.)

2. Make the rillettes: Bring the court bouillon to a boil in a nonreactive skillet large enough to hold the fish snugly in one layer. Add the fresh salmon fillets, reduce the heat, and cover the pan. Simmer until the salmon is opaque in the center, about 8 minutes per inch of thickness. Remove from the heat; set aside to allow the salmon to cool completely in the broth.

3. Drain the poached salmon, remove the skin and any bones, and break up the meat. Place the salmon meat, butter, Armagnac, salt, white pepper, and cayenne in a food processor. Add the smoked salmon. Pulse on and off until the mixture is smooth.

4. Gently fold in the salmon caviar. Adjust the seasonings with additional salt and pepper to taste. Spoon the mixture into a 1-quart decorative mold or serving dish. Tap the mold lightly on a towel-lined surface to eliminate air bubbles. Cover with plastic wrap and chill until set, 8 hours or overnight. The rillettes will keep well for up to 2 days in the refrigerator.

5. Remove the rillettes from the refrigerator 20 minutes prior to serving. Chill a serving spoon under cold water and scoop the salmon onto cold plates.