Light a grill. Skewer the garlic on a 4-inch bamboo skewer. Grill the garlic, tomatillos, tomatoes and onion over a medium-hot fire for 8 to 10 minutes, turning, until charred and softened. Let cool. Transfer to a food processor and pulse until finely chopped. Add the chipotles, adobo sauce, cilantro and sugar and pulse until combined. Season with salt and serve.