Brown the ground beef with diced onions in a large skillet over medium high heat until no longer pink and browned. Drain off the excess grease and return the beef to the skillet.
Stir in the minced garlic and heat for 1 minute until the garlic is aromatic.
In a small bowl, stir together the ketchup, brown sugar and mustard.
Then stir this mixture into ground beef. Stir well to combine all the ingredients. Heat over medium high heat for 2-3 minutes. Remove from heat and set aside.
Heat a separate skillet over medium high heat. Place a tortilla in the skillet.
Add the beef mixture to half the tortilla and top with 1/4 cup of each of the shredded cheeses.
Fold the tortilla in half. Cook until the bottom of the quesadilla is golden brown (2-3 minutes).
Flip and cook until the opposite side is also golden brown (2-3 more minutes) and the cheese is melted.
Remove from the skillet to a separate plate. Follow the same process with the remaining ingredients to form 3 more quesadillas.
Slice each quesadilla into 4 pieces. Serve warm and enjoy!
Notes
Refrigerate the leftovers in an airtight container for up to 4-5 days.