Heat a large skillet over medium heat.
Add the ground beef and cook until browned, breaking it apart as it cooks (about 5–6 minutes).
Drain excess grease if needed.
Add the diced onion and cook for 2–3 minutes until softened.
Stir in the minced garlic and cook for another minute.
Add chili powder, cumin, salt, and black pepper. Stir well to combine.
Remove the beef mixture from heat and set aside.
Lay a tortilla flat on a clean surface.
Sprinkle ¼ cup of shredded cheese over half of the tortilla.
Spoon a few tablespoons of the beef mixture on top of the cheese.
Sprinkle a little more cheese over the beef, then fold the tortilla in half.
Heat a small amount of olive oil in a clean skillet over medium heat.
Place the folded quesadilla in the skillet and cook for 2–3 minutes until golden brown.
Flip and cook for another 2 minutes until crispy and the cheese is melted.
Repeat with the remaining tortillas.
Slice each quesadilla into wedges.
Serve warm with sour cream, salsa, or guacamole.