Cut the tops off the tomatoes and set aside. Use a small knife and spoon to carefully scoop out the seeds of the tomatoes.
Put the cooked quinoa, olive oil, olives, basil and garlic in a mixing bowl and mix well to evenly distribute everything.
Season with salt and pepper to taste, remembering that the olives are salty so you won't need much salt.
Spoon the filling into the tomatoes, pressing it down with the back of a spoon as you go so that the tomatoes are quite firmly packed.
Put the stuffed tomatoes and tomato tops in a roasting dish and cook for 15-20 minutes, or until fragrant, soft and a little caramelised around the edges. The tomatoes will collapse if overcooked, so don't overdo it.