Slice the steak against the grain into thin strips, about 1/4-inch thick. Season with salt and pepper.
Heat 1 tablespoon oil in a large skillet over high heat. Add the steak strips in a single layer and cook without stirring for 2 minutes, until browned. Flip and cook another 1-2 minutes for medium-rare. Transfer to a plate.
Reduce heat to medium-high and add remaining 1 tablespoon oil. Add onion and bell peppers; cook, stirring occasionally, until softened and charred, about 4-5 minutes. Add garlic, chili powder, cumin, and paprika; cook 1 minute more.
Return steak to the skillet and toss with vegetables. Cook for 1 minute to reheat. Remove from heat.
Warm tortillas in a dry skillet or microwave. Fill each tortilla with steak and pepper mixture. Serve with desired toppings.
Tips
Marinate the steak in lime juice, oil, and spices for 30 minutes for extra flavor.
Leftover fajita filling can be stored in the fridge for up to 3 days and reheated in a skillet.
To make it gluten-free, use corn tortillas instead of flour.