Cut each steak in half crosswise. Put in a large resealable plastic bag. Stir together beer, orange juice, lime juice, hot sauce, and garlic, then pour over steaks. Seal bag and refrigerate at least 5 hours and up to overnight.
Prepare a gas or charcoal grill for medium-high heat (see "Set Up Your Grill: Direct Grilling," below). Keep a spray bottle filled with water on hand for any flare-ups.
Drain meat and discard marinade. Blot dry with paper towels. Mix salt and cumin and rub into meat. Spray onions lightly with oil and set, with steaks, on a cleaned and oiled cooking grate. Do not cover. For medium-rare to medium doneness (cut to check), grill steaks 3 to 4 minutes per side if less than 1/2 in. thick; add 1 more minute per side if more than 1/2 in. thick. Turn steaks at least once while grilling (more if juice starts to pool on surface). Grill onions, turning once, until browned on both sides and cooked through, 8 to 10 minutes total.
After removing steaks from grill, let rest 5 minutes (so juices redistribute through the meat) before slicing. With a sharp knife at a slight diagonal, cut across grain into thin strips. Serve meat and onions with tortillas, lime wedges, and hot sauce.
Set up your grill: Direct Grilling
ON CHARCOAL: Ignite 75 to 85 briquets in a chimney starter (or on the fuel grate); open grill vents. When coals are coated with ash, spread into an even layer (if you like, leave a small area empty to create a cooler zone). Check heat: If you can hold your hand 1 or 2 in. above cooking-grate level only 1 to 2 seconds, that's high heat. For medium-high heat, wait until you can hold your hand there only 2 to 3 seconds.
ON GAS: Turn all burners to high, close lid, and heat for 15 minutes. Then adjust to desired heat.
Note: Nutritional analysis is per serving.