Pre-heat oven to 375 degrees. 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 rings and place them on a baking sheet. Place mushroom caps, top side down, on foil rings. (This prevents sogginess.) Bake for 10 minutes.
Remove from oven and spread sauce on mushrooms. Top with spinach, cheese, pepperoni and olives. Bake for an additional 7 minutes, or until cheese has melted. Serve garnished with basil.