Halve the tomatoes and add them to a large zipper bag with the sliced red onion. Add the olive oil, balsamic, parsley, pesto, sugar, garlic and sprinkle with salt and pepper. Seal the bag, getting all the air out. Place into the fridge until ready for dinner.
Place the iceberg lettuce in a large bowl and pour on the tomatoes. Toss and serve! (The dressing from the tomatoes becomes the salad dressing.)