Brush tomatoes and onions with a little olive oil. Grill tomatoes briefly on each side just until warm with grill marks.

Grill onions until tender and browned (they will take longer than the tomatoes).

Combine whole or cut-up tomatoes and onions in a bowl.

Add basil. In a small bowl, combine vinegar, garlic, and remaining oil. Taste and add salt and pepper if necessary.

Combine tomatoes with dressing.

Taste and adjust seasonings if necessary.