Prepare the Bread Topping: Preheat the oven to 200°C (180°C fan) / gas mark 6. Spread the chunks of baguette on a baking sheet, drizzle with 1 tablespoon of melted butter, and season with salt and pepper. Bake for 6 minutes until golden and crispy, then remove and set aside.
Cook the Pasta: Boil the pasta for 2 minutes less than the package instructions to keep it al dente for baking. Drain and set aside.
Start the Cheese Sauce: In a large saucepan, melt 2 tablespoons of butter over medium heat. Add the chopped garlic and mustard powder, cooking for 1 minute while stirring. Sprinkle in the flour and cook for an additional minute.
Add the Milk: Gradually pour in the milk, whisking constantly until the mixture is smooth and free of lumps.
Thicken the Sauce: Simmer the sauce for 5 minutes, whisking frequently until it thickens. Remove from heat and stir in the grated cheddar and half of the Parmesan until melted.
Combine Pasta and Sauce: Season the sauce with salt and pepper to taste. Stir in the cooked pasta until it is evenly coated, then pour into a large ovenproof dish (or individual baking dishes).
Add Toppings and Bake: Scatter the baked bread chunks and the remaining Parmesan over the pasta. Bake for 20 minutes, or until the top is golden and crisp.