Pre-heat the oven to 200ºc,
Heat a large saucepan over high heat then fry the mushrooms in a splash of oil with the garlic.
When the mushrooms are cooked, remove from the pan (along with any of their juices) and set aside.
In the same pot, melt the butter and saute the thyme for 30 seconds then whisk in the flour.
Slowly whisk in the liquid from the mushrooms and the milk until the sauce is smooth. Allow the sauce to simmer for 10 minutes until it's thick and any flouriness has cooked off. You might not need all the milk but different flours absorb liquid differently.
Stir in the mustard, Pecorino and seasoning.
Add the mushrooms and cooked pasta and stir well.
Layer the pasta and mushrooms with the grated mozzarella in an oven-proof casserole dish.
Place in the oven and allow to bake for 15-20 minutes until the top is golden brown.
Remove from the oven and allow to cool for 5 minutes then serve.