Provencal Tuna Sandwiches

Name: Provencal Tuna Sandwiches
Ingredients: 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