In a large skillet over medium heat, add the ground beef and diced onion. Cook until browned and onions are translucent, about 5-7 minutes. Drain excess fat if necessary.
Add the taco seasoning and water to the cooked beef mixture. Stir to combine and let simmer for about 5 minutes.
Preheat your oven to 350°F (175°C). Brush one side of each tortilla with melted salted butter.
In a greased 9x13 inch baking dish, lay down 4 tortillas, buttered side up, overlapping slightly. Spread half of the beef mixture, nacho cheese sauce, diced tomatoes, and cheese over the tortillas.
Add another layer of tortillas, followed by the remaining beef mixture, nacho cheese, tomatoes, and cheese. Top with the final layer of tortillas.
Crush the tostada shells slightly and sprinkle them over the top layer of tortillas for extra crunch.
Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 10-15 minutes, until golden and crispy.
Let it cool for a few minutes. Top with sour cream, shredded lettuce, and additional diced tomatoes before serving.
