1. Preheat the oven to 350 degrees.
2. Remove the stems from the mushrooms and chop them finely. Set aside. Place the mushroom caps in a large baking dish or cast iron skillet. Set aside.
3. Heat 2 tablespoons olive oil in a saute pan over medium heat. Add the sausage, crumbling it with a spatula or wooden spoon. Cook the sausage for 8 to 10 minutes, stirring frequently, until it’s completely browned.
4. Add the chopped mushroom stems and cook for 3 more minutes. Stir in the sage, shallots, garlic, salt and pepper and cook for another 2 minutes, stirring occasionally.
5. Slowly pour in the white wine or chicken stock, stirring vigorously to deglaze the pan (be sure to get up all the browned bits from the bottom of the pan).
6. Add in the mascarpone and parmesan cheese. Continue cooking until the sausage mixture is creamy and the cheese has completely melted.
7. Fill each mushroom cap with the sausage mixture in the well where the mushroom stem used to be (about 1 TBS of the mixture per cap). Arrange the mushrooms in a baking dish large enough to hold all the mushrooms in a single layer (I also like to use my cast iron skillet). Bake for 30 minutes until the stuffing is crusty and the mushrooms have cooked through. Serve warm.
You can prepare the mixture and stuff the mushrooms one day ahead of time. Store in an airtight container in the refrigerator. When ready to cook, place the stuffed mushrooms in a large baking dish or cast iron skillet and bake.