In a small skillet over medium heat, cook the garlic until soft, about 10 to 15 minutes. When cool, peel and place in a food processor or blender along with oil, vinegar, pepper, cinnamon, cloves, oregano and 3/4 tsp. salt. Process until smooth.
Lay the chicken in a Pyrex or non-aluminum baking dish. Cover with half of the marinade and refrigerate for at least 30 minutes or up to 6 hours.
Heat a gas grill to medium-high or light a charcoal fire and let it burn until the coals are covered with gray ash and still very hot.
Brush or spray onions with oil and sprinkle with salt to taste. Lay the onions on the cooler side of the grill, bulbous ends toward the heat. When the onions start to soften and turn golden (about 10 minutes), flip them and cook other side. Remove onions and chop into 2-in. pieces.
Brush or spray oil over both sides of the chicken and lay over the hottest part of the grill. Grill for 6 to 8 minutes per side, turning once. Remove and cut widthwise into thin strips.
Fill tortillas with chicken and onions. Garnish with lime wedges and salsa.
Source: Chef Rick Bayless via People Magazine