In a large saucepan, sautÃ© onion in butter until tender. Remove from the heat. Add the tuna, zucchini, eggs, parsley, lemon juice, seasonings and 1/2 cup bread crumbs. Stir until well combined. Shape into six 1/2-in.-thick patties; coat with remaining bread crumbs.
In a large skillet, heat oil. Cook the patties for 3 minutes on each side or until golden brown.