In a large skillet over medium heat, add the ground beef and diced onion. Cook until the beef is browned and the onions are translucent, about 5-7 minutes. Drain any excess fat. Stir in the taco seasoning and water, and let it simmer for about 5 minutes until the mixture thickens.
Preheat your oven to 350°F (175°C). Grease your 9x13-inch baking dish with melted salted butter to prevent sticking.
Start by placing 4 large flour tortillas at the bottom of the baking dish, overlapping them as necessary to cover the entire bottom of the dish.
Spread half of the beef mixture evenly over the tortillas. Next, drizzle half of the nacho cheese sauce over the beef, followed by half of the diced tomatoes and half of the shredded lettuce.
Layer the tostada shells on top of the first layer for added crunch.
Repeat the layering process with the remaining tortillas, beef mixture, nacho cheese sauce, tomatoes, and lettuce.
Finish with a final layer of tortillas. Drizzle any remaining nacho cheese sauce on top and sprinkle with shredded cheddar cheese if using.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the casserole cool for about 5 minutes before serving. Top with additional diced tomatoes, sour cream, and shredded lettuce for that authentic Crunchwrap experience.