Clean mushrooms and remove stems. Set the caps aside. Chop the stems and then sauté along with the shallots, garlic, rosemary and sage in the olive oil on medium heat until the shallots are translucent, about 5 minutes. Add the spinach, walnuts and nutmeg and cook another minute. Let the mixture cool and then transfer to a food processor, add the miso and process until thoroughly combined and the mixture is the consistency of bread crumbs. Add salt and pepper to suit your taste, place the caps upside down in a casserole dish and spoon the mixture into the caps. Cook at 350⁰ for 30 minutes. Serve with any desired herbs, chopped veggies or seeds as a garnish.