Preheat oven to 400 F. Line a baking sheet with parchment paper.
Remove the stem and scoop out the gills of the mushrooms. Wash gently and pat dry with a paper towel. Brush tops and insides of mushrooms with olive oil.
Spoon 1/4 cup marinara sauce into each mushroom. In medium bowl, mix together the ricotta, egg, spinach and basil. Divide evenly into the four mushrooms.
Sprinkle each mushroom with 1/4 cup of mozzarella. Bake for 20 minutes or until mushrooms are baked through. Enjoy immediately or store in fridge or freezer to enjoy later.
For freezer storage, allow each mushroom to cool completely, wrap in foil then place in larger storage container.