Preheat the oven to 350°F. Grease a half sheet pan (about 18 x 13 inches) generously with cooking spray or melted butter. Set it aside.
Heat a large skillet over medium-high heat and spray with cooking spray. If you're using the optional diced onion, add it to the skillet first and cook for 2 minutes to soften.
Add the ground beef to the skillet, then sprinkle the taco seasoning, smoked paprika, and garlic powder over the top. Mix the spices in as the beef browns, breaking up large pieces with a spoon or spatula.
When the beef is almost cooked through, stir in the tomato paste and a few tablespoons of water. Continue cooking over medium-high heat until the beef is fully cooked through (160°F internal temperature), the mixture looks glossy and saucy, and the meat is well coated.
Arrange 6 of the tortillas around the perimeter of the pan, with about a third of each tortilla hanging over the edge of the pan and the inner edges overlapping in the centre. The bottom of the pan should be fully covered.
Sprinkle 1.5 cups of shredded cheese evenly across the tortilla base.
Spread the taco meat in an even layer over the cheese, all the way to the folded-over tortilla edges.
Top with the remaining 1.5 cups of shredded cheese.
Fold the overhanging tortilla edges back over the cheese layer to seal the perimeter.
Place the last tortilla in the centre on top to cover the small gap where the folded edges meet. The number of tortillas you use will largely depend on the size of your tortillas, but just be sure the bottom and top and sides are completely covered.
Place a second sheet pan directly on top of the assembled quesadillas to weight them down. Bake at 350°F for 9 minutes.
Remove the top pan. Optional but recommended for the most even golden finish: carefully flip the whole sheet pan quesadilla over by sliding it onto a board and back into the pan (top side down).
Bake uncovered for another 9 minutes, until the top is golden and crispy.
Let the sheet pan rest for 3 to 5 minutes, then cut into 8 to 10 squares with a sharp knife.
Serve with salsa, sour cream, guacamole, fresh cilantro, jalapeño, tomato, and lettuce.