Preheat your oven to 400 degrees. Break the stems off of your mushroom tops and clean your tops well. Season your mushroom tops with salt and pepper and bake in the oven for about 8 minutes; set aside. Heat a pan on medium high heat and add lots of butter. Saute your spinach while stirring for about 2 minutes. Add some heavy cream and cream cheese and season with salt, pepper, onion powder, and garlic powder. Cook while stirring for about 6-8 minutes or until nice and smooth. Put some nice sized scoops of your creamed spinach into your mushroom tops and pop them back in the oven for about 3-4 minutes. Plate and enjoy!