Instructions

• Preheat your oven to a cozy 350°F (175°C). Grab your favorite 9x13 inch baking dish and give it a light grease – we don't want anything sticking!
• In a big mixing bowl, gently combine the ground beef, finely diced onion, breadcrumbs, egg, 1/4 cup milk, 1/2 tsp salt, and 1/4 tsp pepper. Now, this is important: mix it just until everything comes together. Overmixing can lead to a tough meatloaf, and nobody wants that! Press this savory mixture evenly into the bottom of your prepared baking dish.
• Pop that meatloaf into the preheated oven for about 20 minutes. This gives it a head start and helps it firm up nicely before we add the cheesy goodness.
• While the meatloaf is baking away, get your macaroni going. Cook it according to the package directions, but here’s a tip: aim for just shy of al dente. It'll finish cooking in the oven, and we want it perfectly tender, not mushy. Drain it well and set it aside, ready for its cheesy bath.
• Now for the star of the show – that luscious cheese sauce! In a medium saucepan, melt the 2 tbsp butter over medium heat. Once it’s shimmering, stir in the 2 tbsp flour. Cook this roux for about a minute, stirring constantly, until it forms a pale, bubbly paste. This is usually where recipes can go wrong if you rush it, so take your time!
• Gradually whisk in the 2 cups of milk, a little at a time, making sure each addition is fully incorporated and smooth before adding more. Keep whisking until the sauce starts to thicken and coats the back of a spoon. Remove it from the heat, then stir in 1.5 cups of the shredded cheddar cheese, the remaining 1/2 tsp salt, 1/4 tsp pepper, paprika, and dry mustard. Stir until the cheese is beautifully melted and the sauce is gloriously smooth and creamy.
• Pour your cooked macaroni into the cheese sauce and give it a good stir to ensure every noodle is coated in that cheesy goodness. Carefully spoon this macaroni and cheese mixture over your partially baked meatloaf layer in the baking dish. For an extra golden crust, sprinkle the remaining 1/2 cup of shredded cheddar cheese right over the top.
• Return the casserole to the oven and bake for an additional 25 minutes, or until the mac and cheese is bubbly and golden brown on top, and the meatloaf is cooked through. You'll know it's ready when your kitchen smells absolutely incredible!