Preheat the oven to 375°F.

Warm a large nonstick skillet over medium-high heat. Add ground beef, onion, bell pepper, and garlic; cook, breaking up the beef into crumbles until no pink remains and the veggies are soft, about 5 minutes. Drain off all grease. Season with the taco seasoning and a bit of salt and pepper; stir to combine.

Pour in water and cook for 2 minutes until thickened, stirring to scrape up any browned bits from the bottom. Remove from heat and set aside.

Lightly brush the bottom and sides of a 9x13 casserole dish with melted butter. Arrange 5 of the 6 tortillas along the edges and bottom of the dish, allowing any excess to hang over the sides and overlapping to completely cover the dish. Sprinkle shredded cheese over the tortillas. Top evenly with the Rotel, then spoon small dollops of the sour cream evenly over the Rotel.

Arrange tostada shells over the sour cream, overlapping them to make them fit.

Spread nacho cheese sauce over tostadas.

Pour the cooked beef mixture over the cheese sauce and spread it into an even layer.

Place the 1 remaining large tortilla in the center of the beef mixture. Wrap the overhanging tortillas around the top tortilla, making sure it's all enclosed.

Brush casserole with the remaining butter and press down any loose ends.

Bake in the preheated oven until golden brown and lightly toasted, about 30 minutes.

Let stand for 5 to 10 minutes and then carefully invert the casserole onto a serving board or cutting board.

Using a large serrated knife, cut the casserole into squares. Top with shredded lettuce, more sour cream or guacamole, and hot sauce if desired.