Heat oven to 200C/fan 180C/gas. Spread the chunks of bread over a baking sheet, drizzle with the melted butter and season. Bake for 6 mins until crispy, then set aside.
Boil the pasta for 2 mins less than stated on the pack. Meanwhile, melt the remaining butter in a saucepan. Add the garlic and mustard, cook for 1 min, then stir in the flour. Cook for 1 min more, then gradually whisk in the milk until you have a lump-free sauce. Simmer for 5 mins, whisking constantly until thickened. Take off the heat, then stir in all the cheddar and half of the Parmesan.
Stir the pasta and some seasoning into the cheesy sauce, then tip into a large ovenproof dish, or 4 individual dishes. Scatter over the bread and remaining Parmesan, then bake for 20 mins until crispy and golden. Can be frozen before baking - defrost thoroughly before cooking.
The last compliancy check was conducted on: 24/08/2017