2 medium white onions, sliced into 1/2-inch rounds (keep the rounds intact for easy grilling)
3 garlic cloves, peeled and roughly chopped
3 tablespoons fresh lime juice
1/4 teaspoon cumin, preferably freshly ground
1 pound beef skirt steak, trimmed of surface fat as well as the thin white membrane called "silverskin"
Vegetable or olive oil for brushing or spritzing the onions and meat
A small bowlful of lime wedges for serving
12 warm, fresh corn tortillas (See how to reheat corn tortillas )