Pre-heat the oven to 400. Line a rimmed baking sheet with aluminum foil and set aside.

Cut or pull the stems off of each mushroom. Dice the stems until you have 1/3 cup diced. Reserve that 1/3 cup and discard the remaining stems. Use a spoon to scoop the dark “gills” out of the inside of each mushroom. Discard the gills and flip each mushroom over so that they are open side down/rounded cap side up. Use a pastry brush to lightly brush the tops of each mushroom with the olive oil. Then flip each mushroom so that the open cup faces up and place them on the prepared baking sheet. Set aside.

Place the sausage in a large skillet or saute pan over medium heat and cook for a few minutes, breaking apart with a wooden spoon until the meat begins to brown a bit. Once the meat starts to break apart, add the onion and garlic and stir together. Continue to cook while breaking up meat and stirring regularly until sausage is cooked through and onions are softened. Add the 1/3 cup of diced mushroom stems you reserved in step 2 along with the fresh spinach and chopped tomatoes. Continue to cook for a couple more minutes until the spinach is wilted.

Transfer the sausage mixture from the pan into a mixing bowl and add the bread crumbs, parmesan cheese, basil, fennel seed, oregano, parsley, salt, pepper and egg white. Stir together until well combined. Spoon the stuffing mixture evenly into each of the prepared mushroom caps from step 2. Place the baking sheet into the oven and bake for 15 minutes or until the stuffing is golden. Mushrooms will shrink back a bit. Serve warm.