Step 1

For the casserole: Preheat the oven to 425°F. Bring a large pot of water to a boil. Season generously with salt. Cook the macaroni until al dente, 7 to 8 minutes. Drain and set aside.

Step 2

In a large Dutch oven, cook the ground beef over medium heat, stirring occasionally to break into small pieces, until browned and cooked through, 7 to 8 minutes. Using a slotted spoon, transfer the beef to a paper towel-lined plate. Set aside.

Step 3

Add the butter to the Dutch oven with the beef drippings and melt over medium heat. Sprinkle in the flour. Cook, whisking constantly, until lightly browned, about 2 minutes. Gradually whisk in the milk. Cook, whisking constantly, until thickened and bubbly, about 5 minutes. Reduce to low heat. Whisk in the mustard, grilling seasoning, Worcestershire, salt, and 2 cups of cheddar cheese, until melted and combined. Stir in the beef and macaroni. Transfer the mixture to a lightly greased 9x13-inch baking dish. Top with the remaining 1 cup of cheddar cheese, then lay the American cheese slices over top to fully cover the casserole. (Tear the pieces as needed.)

Step 4

For the topping. In a small bowl, stir together the panko, butter, sesame seeds, and salt. Sprinkle the mixture evenly over the pasta. Bake until golden brown and bubbly, 15 to 20 minutes. Let cool for 5 minutes.