Campanelle Pasta Salad

1/4 cup extra-virgin olive oil , plus

1/4 cup

1 small or 1/2 medium red onion , chopped

2 cloves garlic , minced

1 (6-ounce) can Italian tuna in oil, such as Flott, drained

1 pint (2 cups) cherry tomatoes, halved

8 ounces frozen artichoke hearts, thawed and quartered

2 tablespoons caper s, rinsed and drained

2 tablespoons chopped fresh thyme leaves

1/4 cup chopped fresh flat-leaf parsley

Kosher salt and freshly ground black pepper