Add 2oz/60g chunks of french baguette to a food processor and pulse until fine breadcrumbs. Melt 1 tbsp butter in a pan over medium heat and stir in your breadcrumbs. Fry for 5 or so mins until golden, shaking the pan frequently and seasoning with a pinch of salt and pepper half way through. Remove from pan.
Add a few sprays of oil to the pan and lay flat 3-4 slices of prosciutto. Fry both sides until deep golden and crispy, then remove and repeat with the rest of your prosciutto. When needed, dice into small shards.
Melt 3 tbsp butter in the pan with the leftover prosciutto fat, then stir in 3 tbsp flour until a paste forms. Gradually pour in 4cups/1litre milk, whisking as you go to ensure no lumps form. Ensure it has thickened before adding cheese, if it's still a very watery consistency the cheese will turn grainy and lose it's flavour.
Meanwhile, cook 14oz/400g pasta in salted boiling water until only JUST cooked. You want it still a bit hard.
Add into the sauce: 1 tbsp dijon mustard, 1 tsp garlic powder, 1/2 tsp onion powder, 1/8 tsp cayenne pepper (optional), nutmeg and salt & pepper to taste. Stir in 9oz/250g brie, 1.5cups/150g comté/gruyére and 1cup/100g mozzarella.
Drain and add pasta along side your crispy prosciutto. Stir to combine, then pour into a baking dish. Top with breadcrumbs and a few sprays of oil, then pop in the oven at 390F/200C for 20-25mins or until golden and bubbly. Serve with a sprinkling of fresh thyme leaves.