Line a sheet pan with foil and arrange the mushroom caps upside down on pan. Dice reserved mushroom stems and set aside.
Add sausage to a medium skillet set over medium heat and cook 4-5 minutes until fully browned, breaking up the meat as it cooks. Once browned, drain off excess grease and transfer to a medium mixing bowl and set aside.
Return the skillet to the heat and melt 1 tablespoon butter. Add mushroom stems, shallots, and fresh thyme and cook 2-3 minutes, stirring frequently, until softened. Transfer to mixing bowl with the cooked sausage. Add cream cheese, Parmesan, mozzarella, 2 tablespoons parsley, onion powder, and black pepper and stir to combine.
Melt remaining 3 tablespoons of butter. Brush the mushrooms with 1 tablespoon of the melted butter. Then pour in the remaining melted butter into the panko breadcrumbs, stirring to combine and until breadcrumbs are moistened.
Using a small spoon or cookie scoop, divide the sausage mixture evenly between mushroom caps. Gently roll or sprinkle each mushroom with the buttered breadcrumbs so they stick to the top.
Bake for 20 minutes, or until the tops are golden brown and mushrooms are cooked through. Sprinkle with parsley if desired and serve immediately.
To Prepare Stuffed Mushrooms in Advance: Prepare the recipe up to baking. Place on a baking sheet and cover with foil. Refrigerate for up to 12 hours (any longer than that, the mushrooms can begin to get soggy.) Remove from the refrigerator while the oven is preheating. Bake for 25 minutes, instead of 20 minutes. Sausage: Use turkey, chicken, or pork Italian sausage, and hot or mid can be used based on preference. Keto/LowCarb/Gluten-Free Stuffed Mushrooms: Omit the panko breadcrumb topping.