Preheat the oven to 500 degrees F.
Bring 5 quarts of salted water to a boil in a stockpot.
Remove the stems from the mushrooms and reserve for another use. Slice the
mushroom caps about 1/4-inch thick. Heat 6 tablespoons of butter in a large
skillet; add the mushrooms and 1/4 teaspoon salt. Sauté over medium heat,
stirring frequently, until they are cooked through, 3 to 5 minutes.
In a large mixing bowl, combine the mushrooms, radicchio, cream, cheeses, sage,
and remaining salt.
Parboil the pasta shells for 4 minutes; drain and add to the ingredients in the
mixing bowl. Toss to combine.
Divide the pasta mixture among 6 to 8 individual, shallow, ceramic gratin dishes
(1 1/2 to 2 cup capacity). Dot with the remaining 2 tablespoons of butter and
bake until bubbly and brown on top, about 7 to 10 minutes.
Garnish with fresh Italian parsley before serving.