1. In a large saucepan, heat butter over medium-high heat. Add onion; cook and stir until tender. Remove from heat.
2. Add tuna, 1/2 cup bread crumbs, zucchini, eggs, parsley, lemon juice, salt and pepper to onion mixture; mix lightly but thoroughly. Shape into six 1/2-in.-thick patties; coat with remaining bread crumbs.
3. In a large skillet, heat oil over medium heat. Add patties; cook 3 minutes on each side or until golden brown and heated through. Yield: 3 servings.