COOK IT.
Rub flank steak with 1 tbsp of olive oil and the BBQ rub. Cover and marinate in the refrigerator for at least 1 hour.
Lightly brush the halved peppers and onion with olive oil.
Preheat your grill to 400°F. Place peppers and onion directly on the grill grates and cook 5 minutes per side. Watch carefully to ensure the peppers and onion do not burn.
Remove peppers and onions from grill and toss lightly with remaining olive oil in a medium sized bowl. Transfer peppers and onions to a cutting board and slice into strips. Set aside.
Place steak directly on hot grill. Cook until medium rare (an internal temperature of 165°F) or until your desired doneness. Pro Tip: open your Pit Boss grill's sear plate for direct flame access.
Remove steak from the grill and transfer to cutting board. Let meat rest for 5 minutes, then slice against the grain into strips.
Place steak, peppers, and onions in a platter and serve immediately with warm tortillas, salsa, guacamole, sour cream, shredded cheese, thinly sliced iceberg lettuce, or your favorite fajita toppings.