Preheat oven to 450°F. Coat a rimmed baking sheet with cooking spray.

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, 1/4 cup 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, 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 remaining sauce to keep warm.

Mound a generous 1/3 cup ricotta filling into each cap and sprinkle with the remaining 1/4 cup Parmesan. Bake until hot, about 10 minutes. Serve with remaining marinara sauce.