Place portobello mushrooms on a baking sheet stem side up. Fill the middle of one mushroom with half of the spinach leaves, ¼ cup of the marinara sauce and 1 cup of the shredded cheese. Top with half of the Italian seasoning then top with half of the sliced cherry tomatoes and half of the olives.
Bake for 15 minutes. Remove from the oven and garnish with Parmesan cheese, basil and more marinara sauce if desired.