Cook the Ground Beef:

1. In a spacious skillet, warm 1 tablespoon of oil over medium heat.
2. Add the chopped onion and cook for 3-4 minutes until softened.
3. Add the ground beef to the skillet, season with salt and pepper, and cook, breaking it up with a spoon, until browned and fully cooked through, about 6-8 minutes.
4. Stir in the tomato paste, chopped spinach (or kale), minced garlic, and chili powder. Cook for another 2-3 minutes until the spinach has wilted and the flavors are well combined.
5. If using, stir in the fresh cilantro for added flavor. Remove from heat and set aside.

Assemble the Quesadillas:

1. Lay out 4 of the flour tortillas on a flat surface. Sprinkle each tortilla with a mix of shredded cheddar and Havarti cheese (about ¼ cup of each cheese per tortilla).
2. Divide the cooked ground beef mixture evenly over the cheese, then top with the remaining tortillas.

Cook the Quesadillas:

1. In a large skillet, heat 1 teaspoon of butter over medium heat. Place one quesadilla in the skillet and cook for 2-3 minutes on each side, or until the tortilla is golden brown and the cheese is fully melted. Add more butter to the skillet as needed between cooking each quesadilla.
2. Repeat the process with the remaining quesadillas.

Serve:

1. Slice the quesadillas into quarters and serve hot with guacamole, sour cream, salsa, and lime wedges on the side for dipping.