Prepare a fire for grilling. Oil grill. Place tomatoes, onion and garlic on the grill. Grill until the skins of tomatoes are charred and tomatoes are softened. Onion should be browned on the outside but still firm. Remove vegetables from grill and peel and chop tomatoes, chop onions and peel and mince garlic. Heat olive oil in a large non-stick skillet and add onion, sauteeing until onions are translucent. Add garlic and cook for 1 minute. Add tomatoes, salt and pepper. Reduce heat to simmer and cook until sauce is reduced to a thick puree, about 30 minutes. Stir in basil and parsley.