Heat a large skillet over medium-high heat. Add the ground beef and break it apart with a wooden spoon, cooking for 5-6 minutes until no pink remains. Drain any excess fat.

Add the taco seasoning, garlic powder, and tomato paste to the beef. Stir to combine and cook for 1-2 minutes until the tomato paste darkens slightly and coats the beef evenly.

Push the beef to one side of the skillet. Add the diced onion and cook for 3-4 minutes until softened. Add the minced garlic and stir everything together. Cook for 30 seconds.

Season with salt and pepper to taste. If using optional add-ins (black beans, corn, bell pepper), stir them in now. Transfer the filling to a bowl and set aside.

Combine the shredded cheddar and Monterey Jack in a bowl. Lay a flour tortilla flat and sprinkle cheese evenly across the surface. Spoon about 1/4 cup of beef filling over one half. Top with a bit more cheese and place a second tortilla on top. Press down gently.

Repeat with remaining tortillas, cheese, and filling to make 4 quesadillas.

Wipe out the skillet and set over medium heat. Add about 1 teaspoon of butter and let it melt. Carefully slide one quesadilla into the skillet and press down gently with a spatula. Cook for 2-3 minutes until the bottom is deep golden brown.

Flip carefully and cook the other side for another 2-3 minutes until golden and the cheese is fully melted.

Transfer to a cutting board and let rest for 1-2 minutes before cutting into halves or wedges. Repeat with remaining quesadillas, adding a touch more butter each time.

Serve hot with guacamole, sour cream, pico de gallo, and lime wedges.