Whisk olive oil, lime juice, chili powder, cumin, paprika, garlic, salt, and pepper in a bowl.
Coat the steak with marinade; let sit 15 minutes while you prep vegetables.
Slice bell pepper and onion; shred cheese and chop cilantro.
Heat a large skillet over medium-high. Sear steak 3 to 4 minutes per side.
Rest steak 5 minutes, then slice thinly against the grain.
Return skillet to medium heat. Add oil and butter; sauté peppers and onions until tender-crisp, 3–4 minutes.
Build quesadillas: tortilla, cheese, steak, vegetables, cilantro, then more cheese; top with another tortilla.
Cook each quesadilla in a lightly oiled skillet until golden and melty, 2–3 minutes per side.
Cut into wedges. Finish with lime juice and serve with sour cream and salsa.