Start by sauteing the diced onion, crushed red pepper flakes, and a pinch of Kosher salt, in olive oil.
Have a large pot ready, on the back burner, with salted water at a boil for the pasta.
Next add in the Italian sausage and cook, on med heat, for about 10-15 minutes until fully cooked.
Using a slotted spoon remove cooked sausage and place in a bowl.
Drain off all the grease from the pan.
Using same pan, on med high heat, add in the butter and saute the sliced Shitake mushrooms and garlic.
Add the sauteed mushroom and garlic to the bowl with the sausage.
Add the sliced sage leaves to the sausage and mushroom mixture and combine.
In another large bowl, add in all the cheeses, sliced radicchio, heavy cream, black pepper, and Kosher salt.
Mix all these ingredients to combine.
Now both bowls have their ingredients and are ready.
Next combine the sausage and mushrooms to the larger bowl with the radicchio and cheese mixture.
Combine all these ingredients together.
At this point add the Rigatoni to the boiling salted water and cook for only 4 minutes.
Drain pasta and add into the bowl with the rest of the ingredients.
Mix gently so not as to break the up the pasta.
Using a spatula, get everything into the buttered baking dish.
Dot the top of the pasta dish with butter.
Place into a 500 degree F. oven for 10-12 minutes, until bubbly, golden brown and crispy on top.