Preheat your oven to 400°F (200°C).
Slice the flank steak into thin strips against the grain.
In a mixing bowl, combine the olive oil, chili powder, cumin, garlic powder, paprika, salt, and black pepper. Mix well.
Add the sliced steak to the bowl and toss to coat the meat evenly with the spice mixture.
On a large baking sheet, arrange the steak strips, sliced bell peppers, and onion in a single layer.
Bake in the preheated oven for 20-25 minutes or until the steak is cooked to your desired level of doneness and the vegetables are tender.
While the fajitas are baking, warm the flour tortillas in a dry skillet over medium heat for about 1 minute per side or until soft and pliable.
Once cooked, remove the sheet pan from the oven. Use a spatula or tongs to mix everything together for a few seconds.
Serve the steak and veggies with the warm tortillas, allowing everyone to build their own fajitas. Add optional toppings as desired.