Rinse fish thoroughly in cold water; pat dry. Place fish in a large stock pot; add sliced onion, salt, pepper, and water to cover. Bring to a boil. Reduce heat, and cook 15 minutes or until fish flakes easily when tested with a fork.
Remove fish to a cutting board; set aside. Strain fish stock, reserving 1 quart; discard onion. Add gelatin to reserved stock; cook over medium heat until gelatin dissolves. Chill until consistency of unbeaten egg white.
Remove and discard skin of fish; flake fish from bone,, discarding bone. Combine flaked fish and next 7 ingredients in a medium mixing bowl; mix well, and fold into gelatin mixture. Spoon into a lightly greased 5 1/2- cup mold; chill until set.
Unmold onto a serving platter. Garnish with parsley sprigs and lemon wedges. Slice and serve with a dollop of mayonnaise.
Oxmoor House Homestyle Recipes