The thin inside skirt steak is ideal here, but flap, flank, hanger, or top round (London broil) also work. Once the carne asada is done, stuff it into tacos, burritos or tortas or serve with salad or rice and beans.
Set up a charcoal grill for direct heat or heat a gas grill over high. (Alternatively, heat a large skillet or grill pan over high heat.)
Combine the oil and lime zest and juice in a nonreactive (glass or ceramic) shallow baking dish. If the steak is in one whole, long piece, cut it with its grain into three shorter pieces. Add the steak to the dish and turn to evenly coat, rubbing the liquids over the meat. Rub the garlic and onion all over the meat, then sprinkle with salt and pepper. Let stand at room temperature while the grill heats or cover with plastic wrap and refrigerate for up to 6 hours.
Place the steaks on the hot grill grate. If using a gas grill, cover them. Cook, flipping the steaks halfway through, until charred and cooked to medium-rare doneness, 2 to 4 minutes total.
Transfer the steaks to a cutting board and let rest for a few minutes. Thinly slice or chop and serve.