1/3 cup extra-virgin olive oil
1/4 cup red wine vinegar
Kosher salt and freshly ground pepper
1/2 small red onion, thinly sliced
4 8-inch-long pieces French or Italian bread, split open
1 clove garlic, halved
3 large plum tomatoes, cored (1 halved, 2 thinly sliced)
2 6-ounce jars tuna in olive oil (preferably Italian), lightly drained
1 red bell pepper, cut into thin strips
1/2 English cucumber, thinly sliced
1/4 cup packed fresh parsley
2 tablespoons capers, drained