1. Preheat oven to 180C
2. Cook pasta in a large pan filled with boiling salted water, for 2 minutes less than it says on the packet (it'll cook more later). Set aside when done.
3. Meanwhile, grate the gruyere and toss in cornflour. Make some hot stock (or bring some homemade to boil) and, either using a microwave or on the hob, whisk the flour/cheese into the boiling broth. Heat some more and it should start to thicken. Add the mascarpone and whisk again, heating through.
4. Fry off first the bacon for a few minutes. Set aside. Now the mushrooms over a very high heat for a few minutes, and take off as soon as they become brown, before the start to exude water. Set aside. Toss the spinach in the same pan, off the heat, it'll wilt in seconds.
5. Add the sauce and bacon, mushrooms and spinach to the macaroni. Tear in the mozzarella and combine. Add mustard and seasoning. Stir again and taste. You'll think it's way too saucy - but that's good, the pasta will soak a lot up remember.
6. Tip into a large baking dish - if it's too much then tip the excess into little oven proof bowls and either freeze or keep in fridge until needed. Top with grated parmesan and bake for 35 minutes or until bubbling and brown.
7. Serve as is. Inwardly sing as your kids eat lots!