Preheat oven to 350 degrees F (175 degrees C).
Hollow out each mushroom cap, reserving scrapings.
Heat a skillet over medium-high heat. Cook and stir sausage, onion, and reserved mushroom scrapings in hot skillet until sausage is browned and cooked through, 4 to 6 minutes. Drain and discard grease; return sausage mixture to skillet.
Stir 3 ounces Parmesan cheese, bread crumbs, garlic, and parsley into sausage mixture. Cook and stir until heated through, 3 to 5 minutes.
Stuff each mushroom cap with sausage mixture and place on a baking sheet.
Bake stuffed mushrooms in the preheated oven for 12 minutes. Sprinkle remaining 1 ounce Parmesan cheese over mushrooms and bake until mushrooms are cooked through and cheese is melted and bubbling, about 3 minutes.
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