Preheat oven to 375; prepare a baking sheet with cooking spray.
Carefully remove stems from mushrooms; don’t discard, set aside.
Put mushrooms cap-side down on baking sheet and bake for 12 to 14 minutes.
Let mushrooms cool a little and then remove water from the caps.
Chop mushroom stems.
In a nonstick skillet with cooking spray, cook mushroom stems, onions, and garlic till softened.
Add spinach to skillet and cook until wilted (if using frozen, just stir in).
In a medium bowl, combine vegan ricotta, nutmeg, salt, vegan parmesan.
Add cooked vegetables to the cheese mixture and stir till well blended.
Fill each mushroom with mixture, then sprinkle with Parma or nutritional yeast.
Return to oven for about 12 to 15 minutes till lightly browned.