Heat a grill or broiler to medium-high. Place tomatoes, chiles, and onion in a single layer and roast, turning from time to time, until skins are slightly charred. 

Have a bowl of ice water ready; plunge the tomatoes into the water to cool, taking care not to allow the tomatoes to burst. 

Place the peppers in a paper bag, seal, and allow to steam for 10 minutes. Peel and discard skin, stems, and seeds (if desired; see note below); do not rinse.

Coarsely chop onion, peppers, and tomatoes, taking care to capture the tomato juice. Place in a nonreactive container and stir in lime juice. Add salt; taste and adjust seasoning. Allow flavors to meld for at least 1 hour; will keep, refrigerated, for up to 4 days