Cook the pasta in salted water according to package instructions, but make sure it is still al dente. It will cook a little bit further in the oven. Rinse under cold water and set aside.
Heat a large skillet with some butter or olive oil and sauté the mushrooms until golden. Make sure most of the moisture is evaporated.
In a pan heat the butter on low heat and once melted add the mascarpone, cream, gorgonzola, grana padano and half of the old cheese. Stir until it is all melted nicely together and forms a thick creamy texture. Add salt and pepper to your taste. Chop the walnuts finely and add to the cream.
Add the pasta and mushrooms and mix well. Put the mixture into an oven dish. Add the remaining cheese on top and put in the oven.
After about 15 minutes check if the top is nice and brown. If not you can put the grill on for a short while or just wait until it is nice and golden.