Quarter and seed tomatoes. Cut the tomatoes into 1/4-inch dice and transfer to a bowl. Wearing rubber gloves, seed and finely chop chiles. Finely chop enough onion to measure 1/4 cup and chop cilantro. Stir chiles, onion, cilantro, and garlic into tomatoes with sugar and lime juice and salt and pepper to taste. Salsa may be made 1 hour ahead and kept at cool room temperature.