Preheat oven to 450°F. Line a baking pan with parchment paper.
Add quinoa and water to a medium pot and bring to a boil. Reduce heat to low, cover the pot, and simmer for approximately 15 minutes. Turn off heat and leave quinoa on stovetop for 5 minutes.
While quinoa is cooking, place mushrooms, open-side up, on prepared pan. Sprinkle with salt and 1/8 teaspoon pepper. Cook for approximately 10 mins, or until mushrooms are tender. Drain excess water and return caps on pan, open-side up.
Mix ricotta, spinach, tomatoes, ¼ cup Parmesan, garlic, Italian seasoning and the remaining 1/8 teaspoon pepper in a medium bowl. Add ricotta mixture into pot of cooked quinoa and combine.
Spoon a generous amount of the quinoa filling into each mushroom cap and sprinkle with remaining ¼ cup Parmesan. Bake at 450°F for about 10 minutes or until hot.