Preheat oven to 350 degrees F. Melt first 5 ingredients together. Place fillets skin side down on broiler pan. Baste with sauce and bake 10 minutes. Base and bake 10 more minutes or until fish flakes to the touch of a fork. Baste and put under broiler for 1 minute. Cover with sliced onion and broil until onion starts to brown (1-2 minutes). Cover onion with tomato slices and broil to warm tomato.