Start by thinly slicing the flank steak against the grain. This ensures tenderness in every bite. Heat one tablespoon of olive oil in a cast iron skillet over medium-high heat. Once hot, add the steak in a single layer (work in batches if needed to avoid overcrowding). Sprinkle half of the taco seasoning over the steak.

Cook the steak for about 1-2 minutes per side, aiming for a medium-rare doneness. Use tongs to flip the pieces for even cooking. Once done, remove the steak from the skillet and set it aside to rest. This resting period helps the juices redistribute for moist and flavorful meat.

Add the remaining olive oil to the skillet, then toss in the sliced onions and bell peppers. Stir-fry the vegetables for 3-4 minutes, just until they soften but still retain some crunch. This keeps that fresh, vibrant texture you expect in fajitas.

Sprinkle the remaining taco seasoning over the veggies and stir until evenly coated. Toss the rested steak back into the skillet and mix it with the cooked vegetables until everything is heated through.

Warm up your tortillas (either in a clean pan or over an open flame), then pile on the steak, vegetables, and your favorite toppings. Serve sizzling hot and enjoy the delicious flavors of your homemade steak fajitas!