Mix together the beer, oil, lime juice, onion, pepper
flakes, cumin, and pepper; this is your marinade.
Place the skirt steak in a large zipper-lock bag, and pour the
marinade over it. Seal
the bag, pressing out the air, and put it in the fridge.
let your steak marinate for a minimum of several hours.
When you're ready to cook, remove your steak from the
bag, reserving a couple of tablespoons of the marinade.
Slice your steak quite thin, across the grain.
Add the oil to a large, heavy skillet over high heat,
and tilt to coat the bottom. When the skillet is good
and hot, add the steak slices, onion, and pepper.
Stir-fry them until the meat is done through and the
vegetables are crisp-tender.
Stir in the reserved marinade, and serve,
topped with guacamole, salsa, and sour cream.