Begin by slicing the flank steak into thin strips against the grain.
In a bowl, mix the olive oil, lime juice, chili powder, cumin, salt, and pepper to create a marinade.
Add the steak strips to the marinade and let them sit for at least 15 minutes.
While the steak marinates, slice the bell pepper and onion into thin strips.
Heat a skillet over medium-high heat until hot.
Add the marinated steak to the skillet and cook for about 5 minutes, stirring occasionally.
Once the steak is browned, add the sliced bell pepper and onion to the skillet.
Cook for an additional 5 minutes until the vegetables are tender.
Remove from heat and taste, adjusting seasoning if necessary.
Serve the fajitas hot with warm tortillas and your choice of toppings.