Preheat your oven to 375 degrees Fahrenheit and prepare a baking sheet (or two, depending on the size) by lining it with parchment paper.
Place the mushrooms hollow side up on the baking sheet.
Cook the Italian sausage in a skillet over medium-high heat until it's browned. Drain any fat and let the sausage cool to room temperature.
Add the cream cheese to a medium bowl, along with 1/2 cup shredded mozzarella, the garlic, the onion powder, salt and chives.
Mix everything together well until the mixture is creamy and smooth, adding the cooled sausage in as well until everything is combined.
Spoon the cheese and sausage mixture into the mushrooms, diving it between them evenly.
Top the mushrooms with the other 1/2 cup shredded mozzarella cheese.
Bake at 375 degrees Fahrenheit for about 15-20 minutes, or until the tops are golden brown and the cheese mixture is bubbly.
Serve while hot.
To make these stuffed mushrooms ahead of time, simply prepare the filling and stuff the mushrooms as directed. Store them in the fridge in an airtight container for up to 24 hours before baking and serving.