In a small bowl, whisk together chili powder, cumin, paprika, garlic powder, onion powder, cayenne pepper, and salt to taste (I like 1 teaspoon).
To a large zipper-top plastic bag, add steak, cilantro, lime juice, and fajita seasoning. Mash until steak is evenly coated. Marinate at least 30 minutes (room temperature) or up to 1 hour (refrigerated).
In a large skillet over medium-high heat, heat oil until shimmering. Add steak and cook, without moving, until crispy and browned on one side, about 3 to 5 minutes. Flip and cook until browned on the second side, about 3 to 5 minutes longer.
Using tongs, stand each piece on a cut side and cook, turning as necessary, until all cut sides are browned and until meat registers 130 to 135 degrees F (for medium rare) or 140 to 145 degrees (for medium), 3 to 8 minutes.
Transfer steak to a cutting board and rest for 5 minutes before cutting into strips (do not discard fat in skillet). Slice steak pieces thin against grain.
Meanwhile, to the skillet with the fat over medium-high heat, add bell peppers and onions and cook until tender, about 5 minutes. Push peppers and onions to one side and add sliced steak back to skillet for serving. Serve with tortillas and your favorite toppings.