Marinate the sliced chicken or beef in olive oil and fajita seasoning for at least 30 minutes.
Heat a skillet over medium-high heat.
Add the marinated meat to the skillet and cook until browned and cooked through.
Remove the meat and set aside.
In the same skillet, add sliced onions and bell peppers.
Sauté the vegetables until they are tender and slightly charred.
Return the cooked meat to the skillet with the vegetables.
Mix everything together and heat through for 2-3 minutes.
Warm the tortillas in a separate pan or microwave.
Serve the fajitas with tortillas, fresh cilantro, sour cream, and lime wedges.