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.
Drain meat and discard marinade.
Mix salt and cumin and rub into meat.
Spray onions lightly with vegetable oil spray and set, with steaks, on a cleaned and oiled cooking grate. Don't cover.
For medium-rare to medium doneness (cut to check), grill steaks 3 to 4 minutes per side if less than 1/2 inches thick; add 1 more minute per side if more than 1/2 inches 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.