Step 1: Preheat the oven and prepare the pasta: Preheat the oven to 2500°F (1125°C). Grease a 125x33cm baking dish and set aside. In a large pot of boiling salted water, cook the pasta for 1 minute less than the al dente time indicated on the package (about 8 minutes). Drain and drizzle with olive oil to prevent sticking.
Step 2: Make the roux: While the pasta is boiling, melt 1/2 cup butter in a large Dutch oven over medium heat. Add 1/2 cup flour and cook, stirring constantly, for 1 minute.
Step 3: Prepare the sauce base: Gradually add 2 cups milk and 2 cups cream. Bring to a boil over medium heat, stirring constantly. Remove from heat.
Step 4: Add flavor and cheese: Stir in 1 teaspoon Worcestershire sauce. Gradually add all of the Velveeta cheese and half (1 1/2 cups) of the cheddar cheese. Stir until the cheese is completely melted and smooth. Add 1 teaspoon salt, 1/2 teaspoon mustard powder, 1/4 teaspoon pepper, and a pinch of nutmeg.
Step 5: Toss and Cook: Add the cooked, drained pasta to the sauce and toss to coat. Transfer the mixture to the prepared baking dish. Sprinkle with the remaining 1 1/2 cups of grated cheddar. Bake for 15 minutes, or until the cheese has melted on the surface and the sauce is bubbling.
Step 6: Grill for finishing (optional): For a crispier, golden-brown surface, place the dish under the grill for 2-5 minutes, watching carefully to avoid burning.
