Thaw frozen fish. Preheat oven to 350 F (moderate). Divide frozen fish into 6 servings. Place in single layer in a baking pan. Mix fat, lemon juice, lemon rind, salt, pepper, and rosemary. Pour over fish. Bake for 25 minutes or until fish flakes easily when tested with a fork. Calories per 2-ounce serving: About 130