Preheat the oven to 350°F or gas mark 4. Line a sheet pan with parchment paper or foil.
In a sauté pan, on medium high heat, heat the olive oil. Add the shallots and cook until translucent and add the spinach and garlic. Cook until the spinach has wilted. Add the goat cheese and sundried tomatoes and cook until the goat cheese has melted. Season with salt and pepper.
Using a spoon scrape the gills out of the mushrooms, place on the the lined sheet pan, spoon 1½ tbsp of tomato sauce inside, and spread evenly. Evenly divide the spinach and goat cheese mixture and place on top of the tomato sauce and top with the grated parmesan cheese.
Bake for 20 minutes or until the the cheese has melted and the mushrooms have softened and release their juices. Serve while still warm.