In a large resealable bag, combine the steak, beer, garlic powder, 2 tsp. salt and 1/4 tsp. pepper. Refrigerate for at least 1 hour, or overnight.
Preheat a grill or grill pan to medium-high. In a bowl, toss together the peppers, onion, 2 tbsp. oil, 1 tsp. salt and 1/4 tsp. pepper. Grill the vegetables, turning occasionally, until tender and slightly charred, 6 to 8 minutes. Transfer to a platter and tent with foil to keep warm.
Remove the steak from the marinade and pat it dry with paper towels. Coat with the remaining 1 tbsp. oil and grill, flipping halfway, 8 minutes for medium-rare. Let rest for 10 minutes.
Thinly slice the steak against the grain and pile it onto the warm tortillas with the peppers and onion. Top with the salsa and cilantro.