Olive oil and some intense flavorings elevate plain old canned tuna. Broiling the bread and finishing with a drizzle of olive oil make this feel like more than just lunch.
Set a rack about 6 inches away from the element and heat the broiler to high.
Add the red onion to a small bowl, toss with 1/4 tsp. salt, and let sit for 5 minutes so it wilts. Toss with the vinegar. Set aside for 10 minutes. Drain the onion and discard the vinegar.
Meanwhile, rub the bread with the cut sides of the garlic. Brush with 1 Tbs. of the olive oil. Sprinkle lightly with salt. Set the bread on a large rimmed baking sheet and broil until toasted but not burnt, 2 to 3 minutes.
In a medium bowl, gently toss the tuna (so the pieces remain coarse) with the parsley, lemon juice, capers, a generous sprinkling of pepper, and the remaining 2 Tbs. oil.
Scatter the onion on the toasts, top with the tuna, drizzle with extra-virgin olive oil, and serve.