Gently remove the stems from the mushrooms and set both the caps and the stems aside.
Lightly spray a large baking sheet with nonstick spray. Place the mushroom caps on the sheet, rounded sides down. Bake in the oven for 12 to 14 minutes. (Afterward, do not turn the oven off.)
Meanwhile, chop mushroom stems. Bring a large skillet sprayed with nonstick spray to medium heat on the stove. Add onion, chopped mushroom stems, and chopped garlic. Cook and stir until softened, about 2 minutes. Add spinach and continue to cook and stir until wilted. Set aside. If needed, pat dry.
Once the mushroom caps are cool enough to handle, pat them dry until free of all excess moisture.
In a medium bowl, combine ricotta cheese, cream cheese, nutmeg, salt, and mozzarella cheese. Mix well. Add the cooked veggie mixture, and stir until blended.
Evenly distribute the cheese-veggie mixture among the mushroom caps. They will be super-stuffed and the mixture will be piled on top, too!
In a small bowl, mix Parm-style topping with garlic powder. Sprinkle the stuffed mushrooms with this mixture.
Bake the stuffed mushrooms in the oven for 8 to 10 minutes, until the topping begins to brown. Let cool slightly and enjoy!!!