Preheat your oven to 240C (470F)
Rub the squash pieces with vegetable oil. Place them skin side down on a parchment-lined tray and roast for about 20 minutes.
In a small saucepan, melt 3½ tablespoons of butter. Add 6 tablespoons of flour and cook over low heat for 1-2 minutes, stirring to form a smooth roux. Gradually whisk in 2 cups of milk until thickened.
Stir in 1 cup of grated cheddar until melted. Mix in the cream, lemon juice, nutmeg, black pepper, and salt if needed. Set aside.
Once the squash is baked, reduce the oven temperature to 200C (400F). Separate the flesh from the skin and blend it into a smooth puree (add a tbsp of water if needed).
Mix about ½ to ⅔ cups of squash puree into the bechamel sauce. Combine with the cooked macaroni and transfer to a 9x13 inch baking dish or few smaller dishes. Top with the remaining cheese and bake for 25-30 minutes.
Melt 2 tablespoons of butter in a pan. Add the breadcrumbs and pumpkin seeds, and toast until golden, stirring continuously.
Sprinkle the crunchy topping over the baked mac and cheese. Garnish with fresh parsley or thyme for a pop of color and fresh flavor.