1medium jalapeño pepper, seeded and finely chopped
1poundlean flank steak, trimmed of all visible fat
2largesweet red bell peppers, seeded and cut into ½-inch-wide strips
1largegreen bell pepper, seeded and cut into ½-inch-wide strips
1large red onion, sliced
Combine the lime juice, beer, oil, garlic, jalapeno, cumin and ½ teaspoon of the salt in a plastic ziplock plastic bag. Add the beef. Squeeze out the air and seal the bag, turning and squeezing to coat the beef. Refrigerate, turning the bag occasionally, at least 2 hours or up to 4 hours.
Spray a grill rack with nonstick spray and preheat the grill. (If you prefer you could cook your fajitas in a grill pan or skillet. It's your call.)
Put the red and green bell peppers and onion in a bowl. Lightly spray the vegetables with nonstick spray and sprinkle with the remaining ½ teaspoon salt; toss to coat. Grill the vegetables over medium heat, turning frequently, until tender, about 8 minutes. Transfer to a bowl and keep warm.
Remove the flank steak from the plastic bag and discard the marinade. Place the steak on the grill rack and grill, turning once, until an instant-read thermometer inserted in the center of the steak, registers 145°F for medium-rare, about 6 to 10 minutes per side. Transfer the meat to a plate and let rest for about 5 minutes.
Thinly slice the flank steak on the diagonal. Toss it with the vegetables.