1. Combine vegetables, dry seasonings, and water in a saucepan or pot. Bring to a boil; simmer 20 minutes, then strain. 2. Cook fish in the strained vegetable stock 6 to 10 minutes, or until fish flakes easily. 3. Remove fish from stock. Arrange on serving platter and cover with plastic wrap. Chill. 4. Strain fish stock and reserve 3/4 cup for horseradish sauce; cool. 5. For horseradish sauce, melt the butter in a saucepan, then blend in flour until smooth, making what the French would call a roux. 6. Add the cooked fish stock gradually, stirring constantly. Cook and stir until the sauce boils and becomes thick and smooth. 7. Remove from heat and stir in horseradish, sugar, salt, sour cream, and eggs. Cool for 15 minutes. 8. Pour the horseradish sauce over the chilled fish, and garnish with shredded lettuce.