In a medium bowl, combine the sliced meat with chili powder, cumin, smoked paprika, salt, pepper, and lime juice. Toss to coat evenly. Let marinate for 10 minutes.
Heat 1 tablespoon oil in a large skillet or cast-iron pan over high heat. Add the meat in a single layer and cook without stirring for 2 minutes, then stir and cook for another 2-3 minutes until browned and cooked through. 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 in spots, about 4-5 minutes. Add garlic and cook for 30 seconds.
Return the meat to the skillet, toss everything together, and cook for 1 minute to reheat. Adjust seasoning if needed.
Serve immediately with warm tortillas and desired toppings.
Tips
For extra flavor, add a splash of beer or broth when returning meat to the pan.
If using chicken, cook until internal temperature reaches 165°F.
Warm tortillas by wrapping in foil and heating in a 350°F oven for 10 minutes, or microwave between damp paper towels for 30 seconds.
Leftover fajita filling can be stored in the fridge for up to 3 days and reheated in a skillet.