Preheat your grill or stovetop pan over medium-high heat.
Season the flank steak with salt, pepper, and fajita seasoning.
Grill the flank steak for about 5-7 minutes on each side.
Remove the steak from the grill and let it rest for 10 minutes.
Meanwhile, heat olive oil in the same pan and add sliced bell peppers and onions.
Cook the vegetables for about 5-7 minutes until tender.
Slice the rested steak against the grain.
In a bowl, layer the sliced steak, sautéed vegetables, diced avocado, and fresh cilantro.
Serve immediately and enjoy!