Preheat your oven to 400°F (200°C).
In a skillet over medium heat, add the ground beef and chopped onion. Cook until the beef is browned and the onions are tender, about 5-7 minutes.
Once the beef is cooked, drain any excess fat. Add taco seasoning, salt, and pepper to the skillet. Stir well and cook for an additional 2-3 minutes.
On a large sheet pan, lay out the 66-inch tortillas and evenly distribute the beef mixture over them.
Spoon the salsa evenly over the beef mixture, then sprinkle the shredded cheddar cheese generously on top.
Place the sheet pan in the oven and bake for 15-20 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven, let cool for a few minutes, slice into portions, and serve with your favorite toppings.