Heat oven to 220C/200C fan/gas 7. Toss the squash with the olive oil and some seasoning, and roast on a baking sheet for 15-20 mins until tender. Meanwhile, cook the macaroni following pack instructions, then drain.
Melt the butter in a pan and stir in the flour and mustard powder to make a paste. Gradually whisk in the milk and simmer to thicken to a smooth sauce, stirring constantly.
Take the sauce off the heat and mash in a third of the squash with the cheddar and half the Parmesan. Season, then stir in the drained macaroni with the remaining squash. Tip into an ovenproof dish, scatter with the remaining Parmesan and bake for 15 mins until golden and bubbling.