Working one by one, cute or pull stems out of each mushroom. After removing stems, use small spoon to gently scrape gills out of mushroom caps, allowing more room for filling. Place hollowed mushroom cap top down on sheet pan.
Repeat process until all 24 large whole baby bella mushrooms have been prepared. Set pan aside.
To Make the Sausage Filling
Preheat oven to 350° Fahrenheit.
Place skillet on stovetop over medium heat. When pan is warm, add 1 tablespoon unsalted butter. Melt butter completely.
Add 1 tablespoon minced garlic and ½ cup diced onion. Stir to incorporate ingredients, then sauté garlic and onion approximately 5 minutes or until onions are translucent, stirring occasionally.
When onions have softened, add 1 pound ground sausage to skillet. Cook sausage, breaking sausage into small pieces with wooden spoon, approximately 8 to 10 minutes or until sausage is completely crumbled and no longer pink.
When sausage is fully cooked, remove skillet from heat and set aside.
Add 4 ounces full-fat cream cheese, 1 large egg, ½ cup shredded parmesan, ½ teaspoon garlic powder, salt, and freshly ground black pepper to medium mixing bowl. Stir ingredients together until well combined.
Transfer sausage, onions, and garlic to mixing bowl. Stir all ingredients together until fully incorporated.
To Assemble the Stuffed Mushrooms
Using scoop with release handle, scoop sausage filling into each mushroom cap, filling caps with approximately 2 tablespoons of filling each. Pack filling into caps evenly to keep mushrooms upright.
Place filled mushrooms on sheet pan in single layer, touching if needed but not stacked or overlapping.
To Bake and Serve the Stuffed Mushrooms
When all mushroom caps have been filled, place sheet pan of stuffed mushrooms in preheated oven. Bake mushrooms 12 to 15 minutes or until mushrooms are browned and filling is heated through.
Remove sheet pan from oven. Transfer mushroom caps to serving tray and set aside to rest 3 to 5 minutes.
After 3 to 5 minutes, sprinkle chopped fresh parsley over tops of sausage stuffed mushrooms. Serve mushrooms warm.
Recipe Notes
Leftovers: Let mushrooms cool completely, then place mushrooms in airtight container and refrigerate. Consume within 4 days. Reheat mushrooms in oven at 350° Fahrenheit, baking until mushrooms and filling are warmed through.