Thaw fish, if frozen. Rinse fish; pat dry with paper towels. On a cutting board, spread each fish open, skin side down. In a small bowl stir together melted butter and lemon juice. Set half of the butter mixture aside. Brush remaining half of the butter mixture over fish. In a small bowl stir together onion, rosemary, lemon peel, salt, and pepper. Sprinkle onion mixture over fish. Fold fish closed. Place fish on a greased 15x10x1-inch baking pan.