Provencal Tuna Sandwiches

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