Preheat Oven & Sheet Pan: Position an oven rack 6 inches below the broiler element. Preheat oven to 450°F (230°C). Place a large rimmed baking sheet inside the oven to heat up while prepping (this ensures an immediate sear on contact).
Slice Steak Against the Grain: Lay the halal flank steak flat and thinly slice at a 45-degree angle across the grain into 1/4-inch thick strips. Slicing against the muscle fibers guarantees maximum tenderness.
Whisk Fajita Marinade: In a large bowl, whisk 2 tablespoons olive oil, fresh lime juice, lime zest, minced garlic, chili powder, cumin, smoked paprika, oregano, garlic powder, salt, and pepper.
Toss Vegetables & Steak: Toss sliced bell peppers and red onions in the bowl with half the marinade. In a separate bowl, toss steak strips with the remaining marinade so juices do not dilute the vegetables.
Roast on Sizzling Pan: Carefully remove the scorching-hot sheet pan from the oven. Drizzle with 1 tablespoon olive oil. Spread peppers and onions in a single layer. Roast for 8-10 minutes until beginning to soften and develop charred edges.
Broil with Steak: Remove pan from oven and scatter seasoned steak strips evenly around and over the peppers in an uncrowded single layer. Switch oven to HIGH BROIL. Broil for 4-5 minutes until steak is caramelized and medium-rare tender and peppers are blistered.
Rest, Garnish & Assemble: Squeeze fresh lime juice over the sizzling pan and scatter with fresh cilantro. Serve immediately right from the sheet pan with warm tortillas, avocado slices, salsa, and sour cream.