These amazing stuffed portobello mushrooms are topped with marinara sauce, spinach, and crispy goat cheese medallions. A delicious and easy vegetarian main course that is sure to be a crowd pleaser! Substitute gluten free breadcrumbs to make gluten free. Note: While you can adjust this recipe to work for any size portobello mushroom, I recommend finding large portobello mushrooms that are roughly 4-inches wide and have slightly deeper caps for filling. This will make them easier to stuff.