Instructions

• Preheat the oven to 375°F (190°C).
• In a large skillet over medium heat, add 1 pound lean ground beef and cook until it begins to brown. Add 2/3 cup diced onion and continue cooking, breaking up the beef, until no pink remains, about 7–10 minutes total.
• Drain excess grease from the skillet. Return the skillet to the heat, add 1 (1-ounce) packet taco seasoning and 1/3 cup water, reduce heat to medium-low, and cook until the mixture thickens, about 2–3 minutes. Remove from heat and set aside.
• Brush the bottom and sides of a 9 x 13-inch baking dish with about half of the 3 tablespoons melted salted butter; reserve the remaining butter for brushing the top later.
• Arrange five of the 8 large flour tortillas in the prepared baking dish so they overlap and hang over the sides. Use one of the five to fill any holes or gaps so the bottom is covered. (Reserve the remaining three tortillas for the top layer.)
• If using the 2 cups shredded cheddar cheese, spread it evenly over the tortillas in the dish.
• Evenly spread 1 1/2 cups deseeded, diced tomatoes over the cheese layer.
• Spread 1 cup sour cream evenly over the tomatoes. To make spreading easier, put the sour cream in a squeeze bottle or a resealable plastic bag with a corner snipped off, or smooth it with a spatula.
• Break the 6 tostada shells as needed and arrange the pieces evenly over the sour cream to cover the surface.
• Spread the cooked beef mixture evenly over the tostada shells. Pour the entire 15-ounce jar of nacho cheese sauce evenly over the beef.
• Place the remaining three tortillas over the beef and cheese to cover as much as possible. Fold the hanging edges of the bottom tortillas toward the center, pressing to seal and eliminate gaps. Brush the top of the assembled casserole with the remaining melted butter.
• Bake uncovered for 20–25 minutes, or until the top tortillas are golden brown.
• Let the casserole rest in the pan for about 5 minutes. Carefully invert the casserole onto a large cutting board. Top with 1 cup shredded lettuce and additional diced tomatoes and/or additional sour cream if desired. Cut into portions and serve immediately.