In a large shallow dish or zipper-top plastic bag, combine beer, lime juice, onion, cilantro, seasoned salt, cumin, 1 teaspoon salt, and 1 teaspoon pepper.
Add steak and turn to coat. Marinate at least 1 hour or up to 2 hours. Let stand at room temperature 30 minutes before grilling.
Preheat grill over high heat for 10 minutes. Clean and oil grate. Add green onions and chiles over direct heat and grill, turning occasionally, until brown spots start to form, about 5 minutes. Push the grilled vegetables to the side.
Remove steak from marinade and shake off excess. Grill steak over direct heat, turning occasionally, according to the thickness of the steak and your preferred doneness temperature: 120 degrees for rare, 130 degrees for medium rare, 140 degrees for medium, 150 degrees for medium well, and 160 degrees for well done.
Transfer steak to a cutting board and let rest for 5 minutes. Slice thinly against the grain. Serve with tortillas, quesadillas, guacamole, grilled green onions, and grilled chiles.