Put the dried porcini mushrooms in a bowl, cover them with boiling water, and cover the bowl.
While the mushrooms soak, make a thick béchamel sauce -- melt 1 tablespoon butter in a saucepan.
Stir in the flour and cook gently for a few minutes.
Whisk the broth and cream into the flour and butter.
Season lightly with salt, pepper, and nutmeg.
Add 2 to 3 tablespoons of the mushroom soaking liquid, being careful not to disturb any sand that has settled to the bottom of the bowl. (The strength of the mushroom liquor will vary; add it to taste.)
Simmer the sauce for 30 minutes or so.
Clean the fresh mushrooms and slice thin.
Lift the dried mushrooms out of the bowl, chop them, and sauté with the other mushrooms in the rest of the butter.
When the mushrooms are nearly cooked, season with salt and pepper and add the garlic and the parsley.
Add the sauce to the mushrooms and taste. If needed, strain in more mushroom liquor.
Preheat the oven to 425°F.
Cook the pasta and add it to the mushroom sauce; taste and correct seasoning.
Put the pasta in a buttered gratin dish and top with grated cheese.
Bake uncovered for about 15 minutes, until the top is crusty and golden.
January 2007 Anne Savarese made this for dinner when I visiting her and Louise, and it was wonderful.