Step 1In a saute pan, heat 2 tablespoons of the olive oil. Add the garlic and cook until aromatic, about 1 minute. Add the tomatoes and season with one-half teaspoon salt and one-eighth-teaspoon pepper. Simmer 5 to 10 minutes until the tomatoes are tender and release their juices. If necessary, add a few tablespoons of water to thin out the sauce slightly. Remove the garlic, adjust seasoning as needed and set aside, covered, in a warm place.
Step 2In a large frying pan, heat the remaining oil over medium heat and saute the onions. Season with 1 teaspoon salt and one-fourth teaspoon pepper and cook until soft and golden, turning with a wooden spoon occasionally to prevent burning. Add the vinegar and simmer until it is just absorbed. Tilt the pan, keeping the onion-flavored oil to one side (you'll use this to cook the tuna). Lift out the onions with a slotted spoon, set aside and keep warm.
Step 3Rinse the tuna and pat dry with paper towels. Season each piece with one-half teaspoon salt. Place the breadcrumbs on a plate, and coat the tuna on all sides. Add a little more oil, if needed, to the pan and place over medium heat. When hot, add the tuna and fry until a deep golden brown crust has formed underneath, about 1 1/2 minutes. Flip the tuna and cook the other side.
Step 4Divide the onions evenly among four plates; place tuna on top of each. Top with the tomato sauce. Scatter with parsley; serve with crispy fried or boiled potatoes.