6 large portobello mushroom caps
3/4 teaspoon kosher salt
1/2 teaspoon freshly ground pepper, divided
1 1/2 cups ricotta cheese
1 1/2 cups fresh spinach, checked and finely chopped
1/2 cup finely shredded Parmesan cheese
3 tablespoons finely chopped kalamata or manzanilla olives
1 teaspoon Italian seasoning
1 cup marinara sauce, homemade or store bought
1/2 cup shredded mozarella cheese
Preheat oven to 450 degrees F. Line a large rimmed baking pan with parchment paper or a silpat.
Peel portobello caps and scrape out the gills using a teasoon. Place mushroom caps, gill-side up, on the prepared pan. Sprinkle with salt and 1/8 teaspoon pepper. Roast until tender, 20 to 25 minutes.
Meanwhile, mash ricotta, spinach, Parmesan, olives, Italian seasoning and the remaining 1/8 teaspoon pepper in a medium bowl.
Place marinara sauce in a small bowl, cover and microwave on High until hot, 30 seconds to 1 1/2 minutes.
When the mushrooms are tender, carefully pour out any liquid accumulated in the caps. Return the caps to the pan gill-side up. Spread 1 tablespoon marinara into each cap; cover the remaining sauce to keep warm. Mound a generous 1/3 cup ricotta filling into each cap and sprinkle with mozarella cheese. Bake until hot, about 10 minutes. Serve with the remaining marinara sauce.