In a large nonstick skillet, combine the first 7 ingredients. Bring to a boil. Reduce heat; cover and simmer until onion is tender, 6-8 minutes. , Arrange fish and peppers over onion mixture. Cover and simmer until fish flakes easily with a fork and peppers are tender, 6-9 minutes. Remove fish and vegetables; keep warm. , Combine cornstarch and water until smooth; gradually stir into pan juices. Bring to a boil; cook and stir until thickened, about 2 minutes. Spoon over fish and vegetables. Serve with lemon.