1. Combine orange juice, beer, oil, lime juice, salt, pepper, oregano, garlic, bay leaves, and rosemary in a glass 9- by 13-in. baking dish. Add orange halves, onion, and steak and mix with your hands to coat evenly. Let stand at room temperature 1 hour, or chill, covered, 2 hours (no longer, or the citrus will "cook" the meat), turning meat occasionally.
2. Heat a grill to very high (550° to 650°). Discard rosemary, bay leaves, and orange halves. Drain meat and onions. Put onions on a doubled sheet of foil and turn up edges just a little to make a shallow 6- by 9-in. container.
3. Grill onions on the foil, stirring occasionally with tongs, until edges start to char, 8 to 10 minutes. Add steak to grill and cook, turning once, until well browned and done the way you like, 8 minutes for medium-rare.
4. Transfer steak and onions to a board; slice steak. Serve with tortillas, guacamole, and hot sauce.
*Warm directly on the grill just until hot, turning once; stack and wrap in a towel.