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Heat the oven to 200C/180C fan/gas 6. Spread the baguette chunks over a baking sheet, drizzle with the melted butter and season. Bake for 6 mins until crisp, then set aside.
Boil the pasta for 2 mins less than stated on the pack, then drain. Meanwhile, melt the butter in a saucepan. Add the garlic and English mustard powder, cook for 1 min, then stir in the plain flour.
Cook for 1 min more, then gradually whisk in the milk until you have a lump-free sauce. Simmer for 5 mins, whisking all the time until thickened. Take off the heat, then stir in the cheddar and half the parmesan.
Stir the pasta and some seasoning into the cheesy sauce. Tip into a large ovenproof dish, or four individual dishes.
Scatter over the bread and the remaining parmesan, then bake for 20 mins until crisp and golden. Can be frozen before baking – defrost thoroughly before cooking.