Step 1: Set up your grill for direct grilling and preheat to medium-high.
Step 2: Char-grill the vegetables on the grill until they are browned on all sides, 8 to 10 minutes for the tomatillos, tomatoes, and onion, and about 4 to 5 minutes for the garlic. (Impale the garlic on toothpicks or a skewer to keep them from falling through the grill grate.)
Step 3: Place the vegetables in a food processor. Add the chipotles, cilantro, salt, and sugar. Puree to a coarse paste, running the processor in short bursts. Correct the seasoning, adding additional salt or sugar.