Optional for serving: cilantrosour cream, salsa, guacamole
Add ground beef, diced onion and minced garlic to a large skillet over medium high heat. Break up beef with a spatula and cook, stirring often, until no longer pink; 5-7 minutes. Drain grease or soak up with paper towels and discard.
To the cooked beef mixture, stir in tomato paste, beef broth or water, and taco seasoning. Bring to a boil, then reduce heat to medium-low and cook, stirring often, until sauce is thickened and reduced; about 10-15 minutes.
Spread butter onto one side of each tortilla.
Heat another large skillet over medium heat and add one tortilla, butter side down. Sprinkle cheese on one half of the tortilla, then layer a fourth of the beef mixture, a few slices of avocado, and more cheese.
Fold the tortilla in half, and cook until golden on the bottom. Carefully use a flat spatula to flip the quesadilla over and cook the other side until golden. Gently press down to cook evenly.
Transfer to a wire rack or cutting board while you cook the other quesadillas.
If making a large batch, keep warm in the oven. Serve with salsa, sour cream and guacamole.
Feel free to adjust the amount of cheese to use more or less to your liking. Use your favorite kind of cheese.