1
Preheat the Oven. Preheat your oven to 240°C (465°F).
2
Roast the Squash. Rub the squash pieces with vegetable oil. Place them skin side down on a parchment-lined tray, spread out rather than touching, and roast for about 20 minutes, until a knife goes through the thickest part easily and the cut edges have browned.
3
Make the Bechamel Sauce. In a small saucepan, melt 50 g of butter. Add 50 g of plain flour and cook over low heat for 1-2 minutes, stirring to form a smooth roux. Keep it pale. Gradually whisk in 500 ml of milk, a little at a time, letting each addition come together before the next goes in.
4
Combine with Cheese. Take the pan off the heat, then stir in 100 g of the finely grated cheddar until it melts. Mix in the cream, lemon juice, nutmeg, black pepper, and salt if needed. Stir as little as you can once the cheese is in: heat and stirring are what turn a cheese sauce grainy. Set aside.
5
Blend the Squash. Once the squash is baked, reduce the oven temperature to 200°C (400°F). Separate the flesh from the skin while it is still warm and blend it into a smooth puree (add a tbsp of water if needed).
6
Assemble the Dish. Mix about 120 to 160 g of the squash puree into the bechamel sauce. Combine with the cooked macaroni and transfer to a 23 x 33 cm (9 x 13 inch) baking dish, or a few smaller ones. Keep the sauce looser than looks right, because the pasta keeps drinking it in the oven. Top with the remaining cheese and bake for 25-30 minutes.
7
Prepare the Topping. Melt 2 tablespoons of butter in a pan. Add the breadcrumbs and pumpkin seeds, and toast until golden, stirring continuously.
8
Serve and Enjoy. Sprinkle the crunchy topping over the baked mac and cheese just before it goes to the table, so it stays crisp. Garnish with fresh parsley or thyme.