Preheat oven to 350°F.
Clean the mushrooms by wiping the tops of each mushroom with a damp paper towl. Remove stems from mushrooms by gently twisting and pulling, or by cutting them out with a knife. Place mushrooms on a parchment lined baking sheet, gill sides up. Lightly sprinkle the mushrooms with salt. Set aside.
In a skillet set over medium to medium-low heat, add olive oil and the diced onion. Cook until translucent and slightly golden, about 7-8 minutes. Transfer to a large bowl, set aside.
Add Italian sausage to the now empty skillet. Cook and break apart until browned. Once cooked through, drain the grease. Add garlic clove to the sausage and cook 1 minute more. Use a paper towl and (carefully!!) dab any excess grease from the sausage. Transfer the sausage to the bowl with the onion.
Add cream cheese, chopped parsley, and pepper to the sausage and onion bowl. Mix to combine.
Spoon (or pipe) mixture into the mushrooms.
Sprinkle the breadcrumbs over the tops of the filled mushrooms.
Cook mushrooms at 350°F for 15-18 minutes or until warmed and golden.
Remove from the oven and sprinkle with additional fresh parsley, if desired. Serve warm.