roasting. Roast the tomatoes on a baking sheet 4 inches below a very hot broiler until they're darkly roasted (they'll be blackened in spots), about 6 minutes. Flip them over and roast the other side 5 to 6 minutes more will give you splotchy-black and blistered tomatoes that are soft and cooked through. Cool. Working over the baking sheet, pull off and discard the blackened skins; for round tomatoes, cut out the hard cores where the stems were attached.
Roast the chiles and garlic in a dry skillet or on a griddle over medium heat, turning them occasionally, until they are soft and darkened in places, about 5 minutes for the chiles, 15 minutes for the garlic. Cool, then slip the papery skins off the garlic.
Finishing the salsa. Either crush the roasted garlic and chiles to a smooth paste in a mortar or chop them to near-paste in a food processor. If using a mortar, crush in the tomatoes one at a time, working them into a coarse puree. For a food processor, add the tomatoes and pulse to achieve a coarse puree.
Scoop the chopped onion into a strainer and rinse under cold water. Shake to remove the excess moisture.
Transfer the salsa to a bowl, stir in the onion and cilantro and season with salt, usually a generous l/2 teaspoon. Thin with a little water (usually about 2 tablespoons) to give it a spoonable consistency. Perk it all up with vinegar or lime if you wish.
working ahead: Once the onion and cilantro have been added to the salsa, it should be eaten within a few hours. Without onion and cilantro, the refrigerated salsa base keeps for several days, though the flavors will dull