Combine first 10 ingredients in a large skillet. Cook over medium heat 15 minutes or until vegetables are tender.
Rinse fish thoroughly in cold water; pat dry, and place in a greased 13- x 9- x 2-inch baking pan. Brush fish with 1 tablespoon olive oil. Spoon vegetable mixture over fish. Bake, uncovered, at 325° for 45 minutes or until fish flakes easily when tested with a fork.
Transfer fish to a serving platter; spoon vegetables around fish. Garnish with watercress, if desired. Serve immediately.