Preheat the oven to 375 degrees F and line a sheet pan with foil.
Using a damp towel, brush the dirt off the mushrooms. Remove the stems from the mushrooms and place the mushroom caps, cavity-side up on the pan. Finely dice the mushroom stems to use in the filling.
Add sausage to a medium skillet and saute until fully browned, over medium heat, breaking up the meat as it cooks. Once browned, drain off excess grease transfer the sausage to a mixing bowl, and return the skillet to the heat.
Over medium heat, melt 1 tablespoon of butter. Once the butter has melted, add mushroom stems, shallots, and fresh thyme and cook for 2-3 minutes, stirring frequently, until the shallots have softened. Remove from the heat and add this mixture to the mixing bowl with the cooked sausage.
Add cream cheese, parmesan, mozzarella, 2 tablespoons parsley, onion powder, and black pepper to the sausage mixture and stir to combine everything evenly together.
Melt the remaining 3 tablespoons of butter in a skillet or in the microwave. Brush the cavity of the mushroom caps with the butter, using about 1 tablespoon. Combine the remaining melted butter with the panko breadcrumb in a small mixing bowl, stirring until the breadcrumbs are moistened.
Using a small spoon or cookie scoop, divide the sausage mixture evenly between mushroom caps. Sprinkle the buttered breadcrumbs over the filling and use your fingers to help adhere the breadcrumbs to the mixture.
Bake for 20 minutes, or until the tops are golden brown and mushrooms are cooked through. Sprinkle with parsley if desired and serve immediately.
Video
Notes
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.