Heat oven to 425 degrees. Remove stems from mushrooms and reserve. Clean mushrooms and place curved side up on 1 large or 2 small baking sheets. Brush with olive oil, sprinkle with 1/8 tsp of the salt and bake at 425 degrees for 10 minutes. Drain off any liquid and turn over mushrooms.Advertisement
Chop enough of the stems to equal 1 cup. Heat a large nonstick skillet over medium-high heat. Crumble sausage into skillet and cook 3 minutes, breaking apart with a wooden spoon or silicone spatula. Stir in chopped mushroom stems and cook 3 minutes. Add spinach and sage. Cook 1 minute, remove from heat and transfer to a bowl.
Stir in bread crumbs, 4 tbsp of the Parmesan, remaining 1/4 tsp salt and the pepper. Mix in egg. Using a spoon and your hands to compress filling slightly, divide filling among mushroom caps. Sprinkle with remaining 1 tbsp Parmesan and bake at 425 degrees for 10 to 12 minutes, until tops are lightly browned. To make ahead: Prepare filling and stuff mushrooms. Refrigerate, then bake for 20 minutes before serving.