Make court bouillon: Cut up leek, carrot, and celery and in a large saucepan bring to a boil with remaining ingredients. Boil mixture 10 minutes and pour through a fine sieve into a bowl. Court bouillon keeps, covered and chilled, 3 days or, frozen, 2 months. Makes about 2 quarts.
Rinse fillets under cold water then pat dry. Lay salmon strips on top of sole fillets. Coat lightly with 2 tablespoons of the butter on one side. Sprinkle on the same side with 1 teaspoon salt and 1/4 teaspoon white pepper, then add the parsley.
Starting at the narrow tapered end, roll up each fillet loosely and secure the ends with 1 or 2 toothpicks. This may be prepared in advance, covered with plastic wrap and refrigerated up to 4 hours.
Bring court bouillon to a simmer. Place fish in simmering court bouillon for 5 to 8 minutes, or pour boiling Court Bouillon over fish and let stand 10 to 12 minutes. Transfer fish rolls one at a time with a small metal spatula to plates. Let rest 1 to 2 minutes, then remove the liquid that accumulates.