Preheat oven to 400 degrees. Place a wire rack on a rimmed baking sheet and set aside.
With a spoon, gently scoop out the gills inside the mushrooms. Discard the gills and lightly brush the outside of the mushrooms with olive oil. Set mushrooms on wire rack and season with a pinch of salt for each.
In a medium bowl, add the ricotta cheese, egg, drained spinach, Parmesan cheese, basil, garlic powder, red pepper flakes, and pinch of nutmeg. Stir until well combined.
Divide the ricotta filling between the Portobello mushrooms. Top each mushroom with 1⁄4 cup marinara sauce and a sprinkle of Parmesan cheese, if desired.
Bake mushrooms on wire rack for 20-25 minutes or until the cheese is melted and mushrooms are tender. If you want to brown the cheese on top, put the pan under the broiler for 1 minute. Be careful as the cheese can brown quickly.
Remove from the oven and garnish with fresh basil leaves, if desired. Serve warm.