Cook the Beef:Heat olive oil in a large skillet over medium heat. Add ground beef and cook until browned. Drain excess fat if necessary.
Add Flavor:Stir in chopped onion and cook for 3 minutes. Add garlic and cook for 1 more minute. Mix in chili powder, cumin, paprika, salt, pepper, and tomato sauce. Simmer for 3-4 minutes. Remove from heat.
Assemble the Quesadillas:Lay out tortillas and sprinkle a layer of cheddar and Monterey Jack cheese on one half of each tortilla. Spoon the beef mixture over the cheese, then top with more cheese. Fold tortillas in half to seal.
Cook the Quesadillas:Heat a clean skillet over medium heat. Lightly butter or oil the pan, then cook each quesadilla for 2-3 minutes per side, or until golden and crispy. Flip carefully.
Slice and Serve:Let quesadillas cool for a minute, then slice into wedges. Serve with sour cream, salsa, or guacamole if desired.