1. Cook the macaroni in a large saucepan of boiling salted water for 8–10 minutes. Drain well and set aside.
2. Melt the butter over a medium heat. Add the flour and stir to form a roux, cooking for a few minutes. Gradually whisk in the milk, a little at a time. Cook for 10–15 minutes until thickened and smooth.
3. Meanwhile, preheat the grill to hot. Remove the sauce from the hob, add 175g of the cheese and stir until it is well combined and melted.
4. Add the macaroni to the sauce and mix well. Transfer to a deep ovenproof dish. Sprinkle over the remaining cheddar and the parmesan and place the dish under the hot grill. Cook until the cheese is browned. Serve immediately.