01 - Preheat oven to 400°F. Line a baking sheet with parchment paper for easy cleanup.
02 - Brush both sides of the portobello caps with olive oil and season with salt and pepper. Arrange gill-side up on the prepared baking sheet.
03 - In a medium bowl, combine ricotta, mozzarella, Parmesan, egg, basil, garlic, oregano, salt, and pepper. Mix until smooth and fully incorporated.
04 - Spoon a thin layer of marinara sauce into each mushroom cap, using approximately 1 tablespoon per cap.
05 - Divide the ricotta filling evenly among the mushroom caps, mounding slightly if necessary.
06 - Spoon the remaining marinara sauce over the ricotta filling in each cap.
07 - Sprinkle shredded mozzarella and grated Parmesan generously over each stuffed mushroom.
08 - Bake for 25 to 30 minutes, until mushrooms are tender and cheese is golden brown and bubbly.
09 - Let cool for 5 minutes before serving. Garnish with fresh parsley or basil if desired.