1. Sauté all ingredients, except the bread crumbs, walnuts, and mustard, in the olive oil for 15 minutes.
2. Mix with the remaining ingredients, except for half the mustard.
3. Transfer to an oiled baking dish and top with the remaining mustard.
4. Bake for one hour in a preheated 375° oven. Cook covered for more tender mushrooms and uncovered if you prefer crisper mushrooms.