In a skillet over medium heat, cook the ground beef until browned and cooked through, about 7-8 minutes. Drain excess fat.
Warm the large flour tortillas in a dry pan or microwave until pliable.
On each tortilla, spread a quarter cup of refried beans in the center.
Add a layer of cooked ground beef on top of the beans.
Drizzle about 1/4 cup nacho cheese sauce over the beef.
Place one tostada shell on top of the cheese sauce to add crunch.
Spread a tablespoon of sour cream over the tostada shell.
Add shredded lettuce, diced tomatoes, and shredded cheddar cheese over the sour cream.
Fold the edges of the tortilla up and over the fillings to form a hexagonal wrap.
Place the wrap seam-side down on a hot skillet over medium heat.
Cook for 2-3 minutes until the tortilla is golden brown and crispy, then carefully flip and cook the other side for another 2-3 minutes.
Remove from the pan, cut in half to reveal the layers, and serve warm.