Preheat oven to 375˚F. Remove mushroom stems and scrape out the dark gills under the mushroom caps with a spoon. Lightly coat mushroom caps with oil.
Form 4 small aluminum foil circles and place them on a baking sheet. Place mushroom caps topside down on foil circles. (This prevents sogginess.) Bake for 10 minutes
Remove mushrooms from oven and spread them with sauce. Top with spinach, cheese, pepperoni, and olives. Bake for an additional 7 minutes, or until cheese has melted. Garnish with basil and serve.