Heat a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until browned and crumbly, about 5 minutes.

Transfer the beef to a colander to drain excess fat. Carefully wipe the skillet clean.

Return the skillet to medium heat and add the vegetable or olive oil. Add the chopped onion, season with salt and pepper, and cook for about 3 minutes until softened.

Stir in the tomato paste, chopped spinach, minced garlic, and chili powder. Cook for about 1 minute, until the spinach wilts and the mixture is well combined.

Return the drained beef to the skillet and stir until evenly blended. Stir in the cilantro if using.

Transfer the beef mixture to a bowl. Wash and dry the skillet, or use a clean skillet.

Combine the shredded cheddar and havarti in a separate bowl.

Place the skillet over medium heat and add ½ teaspoon of butter. Once melted, place one tortilla in the pan.

Sprinkle 2 tablespoons of the cheese mixture over half of the tortilla. Add 1/8 of the beef mixture, then top with another 2 tablespoons of cheese.

Fold the bare half of the tortilla over the filling, cover the pan, and cook for about 2 minutes until the bottom is golden and the cheese begins to melt.

Flip the quesadilla and cook uncovered for another 2–3 minutes until the underside is browned and the cheese is fully melted.

Transfer to a cutting board and let rest for 1 minute before slicing into wedges.

Repeat with remaining butter, tortillas, cheese, and beef mixture.

Serve warm with guacamole, sour cream, salsa, and lime wedges.