Preparation of Oven and Mushrooms: Commence by preheating the oven to 350 degrees Fahrenheit. Proceed to wash the mushrooms gently and dry them thoroughly with paper towels.
Sautéing Sausage and Onions: In a large skillet, heat the olive oil and melt the butter over medium heat. Add the ground Italian sausage to the skillet and brown it. When the sausage is approximately halfway cooked, incorporate the finely chopped sweet onion and continue cooking for several minutes.
Herb and Spice Incorporation: Reduce the heat to low. Add the minced garlic, marjoram, kosher salt, and freshly ground black pepper. Cook this mixture for 1 minute, ensuring constant stirring to prevent burning. Following this, remove the skillet from the heat and allow the mixture to cool.
Filling Preparation: Transfer the sausage mixture to a large bowl. Add the softened cream cheese, Parmesan cheese, breadcrumbs, and 1 ½ tablespoons of the chopped fresh parsley. Stir the ingredients together until thoroughly combined. Subsequently, spoon or scoop the sausage mixture into the mushroom caps.
Baking: Sprinkle the filled mushrooms with the remaining parsley. Bake them uncovered for 20 minutes, or until they are heated through and slightly golden on top.