Preheat a gas grill to high. (If cooking over charcoal, allow the coals to burn until they are covered with gray ash.) Grill the tomatoes and onion, turning frequently, for 10 to 15 minutes or until the vegetables are lightly charred but still firm inside. Grill the chili(es) until blackened on the outside. (If necessary, cover the grill periodically for 1 minute to increase the heat and help the vegetables char.)
Coarsely chop the tomatoes and onion. Peel the skin from the chili(es), seed the chili(es), and chop. Combine the vegetables in a bowl with the cilantro, lime juice, and salt. Serve at room temperature, or cover and refrigerate for up to 2 days.
Rainy-Day Method: Heat the broiler and cook the tomatoes, onion, and chili(es) about 4 inches from the heat, turning frequently, until lightly charred but still firm on the inside. Proceed as above.