Creamy mac and cheese au gratin

How to make a creamy macaroni and cheese gratin:
Step 1: Preheat the oven and prepare the pasta: Preheat the oven to 122°C (gas mark 6). Grease a 125 × 33 cm baking dish and set aside. In a large pot of boiling salted water, cook the pasta one minute less than the time indicated on the package for al dente cooking (about 8 minutes). Drain and toss with a drizzle of olive oil to prevent sticking.
Step 2: Preparing the roux: While the pasta is cooking, melt 125g of butter in a large cast iron casserole dish over medium heat. Add 125g of flour and cook, whisking constantly, for 1 minute.
Step 3: Preparation of the sauce base: Very gradually whisk in 500 ml of milk and 500 ml of light cream. Bring to a boil over medium heat, whisking constantly. Remove from heat.
Step 4: Seasoning and cheese: Whisk in 1 teaspoon of 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 the sauce is smooth. Stir in 1 teaspoon salt, 1/2 teaspoon dry mustard, 1/4 teaspoon pepper, and a pinch of nutmeg.
Step 5: Toss and bake: Add the cooked, drained pasta to the sauce and mix well. Transfer the mixture to the prepared casserole dish. Sprinkle with remaining shredded cheddar cheese (1 1/2 cups). Bake for 15 minutes, or until the cheese is melted and the sauce is bubbling.
Step 6: Gratin for Finish (optional): For a crispier, browner surface, place the dish under the broiler for 2 to 5 minutes, watching carefully to avoid burning.