Step 1: Preheat and Prep: Preheat oven to 375F (190C). Lightly grease a 9x13 inch baking dish.
Step 2: Prepare the Meatloaf Mixture: In a large bowl, combine ground beef, breadcrumbs, egg, milk, onion, garlic, Worcestershire sauce, thyme, salt, and pepper. Mix gently with hands until just combined. Do not overmix.
Step 3: Form the Meatloaf Base: Press the meatloaf mixture evenly into the prepared baking dish, creating a solid base.
Step 4: Bake the Meatloaf: Bake the meatloaf base in the preheated oven for 25 minutes, or until it is partially cooked and slightly firm.
Step 5: Cook the Macaroni: While the meatloaf bakes, cook the macaroni according to package directions until al dente. Drain well.
Step 6: Prepare the Cheese Sauce: In a large saucepan, melt butter over medium heat. Whisk in flour until smooth to create a roux. Gradually whisk in milk until smooth and thickened.
Step 7: Add Cheese and Seasoning: Remove from heat and stir in 3 cups of shredded cheddar cheese until melted and smooth. Season with salt and pepper.
Step 8: Combine Macaroni and Cheese Sauce: Add the cooked and drained macaroni to the cheese sauce. Stir to coat evenly.
Step 9: Assemble the Casserole: Remove the partially baked meatloaf from the oven. Spread the macaroni and cheese mixture evenly over the meatloaf base.
Step 10: Top with Remaining Cheese: Sprinkle the remaining 1 cup of shredded cheddar cheese evenly over the macaroni and cheese layer.
Step 11: Bake the Casserole: Return the casserole to the oven and bake for an additional 25 minutes, or until the cheese is melted, bubbly, and lightly golden brown.
Step 12: Rest and Serve: Let the casserole rest for 10 minutes before slicing and serving. This allows the casserole to set slightly, making it easier to slice.
Optional Tips: For a crispy topping, sprinkle breadcrumbs mixed with melted butter over the cheese before the final bake. To add vegetables, stir cooked peas or corn into the macaroni and cheese mixture. Ground turkey can be substituted for ground beef. Use your favorite cheese blend instead of cheddar.