01 - In a mixing bowl, combine olive oil, smoked paprika, chili powder, cumin, garlic powder, cayenne, salt, and black pepper. Add chicken breasts and coat thoroughly with the marinade.
02 - Preheat grill or grill pan to medium-high heat. Place marinated chicken on the grill and cook for 5 to 6 minutes per side, until internal temperature reaches 165°F and juices run clear. Remove and let rest for several minutes, then slice or dice into thin pieces.
03 - While chicken cooks, thinly slice bell pepper and red onion, and shred the Monterey Jack and cheddar cheeses. Slice the pickled jalapeños and chop the cilantro.
04 - Place one tortilla on a clean surface. Evenly distribute a portion of both cheeses over half of the tortilla, then layer with grilled chicken, red bell pepper, red onion, jalapeños, and cilantro. Fold the tortilla in half over the fillings.
05 - Heat butter or neutral oil in a skillet over medium heat. Add assembled quesadilla and cook for 2 to 3 minutes on each side, pressing gently, until the tortilla is crisp and golden and cheese has melted. Repeat with remaining tortillas and fillings.
06 - Transfer grilled quesadillas to a cutting board. Allow to cool briefly, then slice each into wedges. Serve immediately, accompanied by salsa, guacamole, and sour cream if desired.