Step 1: Preheat the Oven and Cook Pasta

• Preheat your oven to 375°F (190°C).
• Bring a large pot of salted water to a boil. Add the pasta and cook according to the package instructions until al dente. Drain and set aside.

Step 2: Prepare the Beef Mixture

• In a large skillet, heat a splash of oil over medium heat. Add the chopped onion and sauté for about 5 minutes, or until softened and translucent.
• Add the ground beef, breaking it up with a spoon, and cook until browned. Season with salt and pepper.
• Stir in the minced garlic, diced tomatoes, and sliced leeks. Let the mixture simmer for 10 minutes, allowing the flavors to meld.

Step 3: Make the Cheese Sauce

• In a separate saucepan, melt the creamed butter over medium heat. Add the flour, stirring constantly for about 2 minutes to create a smooth roux.
• Gradually whisk in the milk, ensuring no lumps form. Continue whisking until the sauce thickens.
• Stir in the grated Parmesan cheese and a pinch of nutmeg. Mix until the cheese melts and the sauce becomes smooth. Season with salt and pepper.

Step 4: Combine and Assemble

• In a large mixing bowl, combine the cooked pasta with the beef mixture, ensuring everything is evenly distributed.
• Transfer the pasta-beef mixture to a baking dish. Pour the cheese sauce over the top, spreading it evenly.
• Sprinkle the grated cheddar, mozzarella, or cheese blend over the surface of the dish.

Step 5: Bake

• Place the baking dish in the preheated oven. Bake for 25-30 minutes, or until the cheese topping is golden and bubbly.

Step 6: Serve

• Remove the dish from the oven and let it rest for a few minutes. Serve hot with a fresh salad or steamed vegetables on the side.