01 - Heat olive oil in a large skillet over medium heat. Add onion and cook until softened, about 3 minutes. Add garlic and cook for 30 seconds.
02 - Add ground beef, breaking up with a spatula, and cook until browned and cooked through, about 5–6 minutes. Drain excess fat if necessary.
03 - Stir in cumin, chili powder, smoked paprika, salt, and black pepper. Cook for 1–2 minutes until fragrant, then remove from heat.
04 - Combine refried beans with lime juice and chopped cilantro in a small bowl.
05 - Lay tortillas flat. Spread 2 tablespoons of bean mixture over half of each tortilla. Layer with beef mixture, then sprinkle shredded cheddar and Monterey Jack cheeses evenly.
06 - Fold tortillas in half to enclose fillings.
07 - Heat a large nonstick skillet or griddle over medium heat. Brush outside of each quesadilla lightly with softened butter.
08 - Cook quesadillas in batches for 2–3 minutes per side until golden brown and cheese is melted. Remove and let rest for 1 minute before slicing.
09 - Serve warm with salsa, sour cream, sliced jalapeños, and fresh cilantro as desired.