1. Heat the oil in a large nonstick skillet over medium heat. Add the onions and cook, stirring occasionally, until translucent, about
10 minutes. Add the bell pepper and cook, stirring occasionally, until the onions are browned and very soft, about 20 minutes. Transfer the onion mixture to a medium bowl; set aside.
2. Sprinkle the tuna with the salt and ground pepper. Heat the same skillet over medium-high heat. Add the tuna and cook until lightly browned, about 3 minutes. Turn the tuna and top each steak with one-quarter of the onion mixture. Add the wine and parsley and cook, covered, until the tuna is pink in the center, 2-3 minutes.