1. Preheat broiler with oven rack 5 inches from heat. Line a baking sheet with aluminum foil.
2. Place garlic, tomatoes, and onion, cut sides down, on prepared baking sheet. Broil 5 minutes or until skins are blackened. Set aside.
3. Wipe chiles clean with a damp cloth. Remove and discard seeds and stems. Peel garlic.
4. Heat oil in a skillet over medium heat. Add chiles and cook, turning often, 2 minutes or until dark and toasted. (Do not burn.) Transfer chiles to a bowl; cover with 1 cup boiling water. Let stand 20 minutes or until softened.
5. Transfer chiles to a food processor with a slotted spoon, reserving soaking liquid. Add reserved garlic, tomatoes, and onion. Pulse until finely chopped. Add salt, cumin, and lime juice; pulse until well blended. Add reserved soaking liquid if necessary to achieve desired consistency. Cover and chill.