Place the Italian sausage in a medium sauté pan over medium heat. Cook for 4–5 minutes or until the sausage is no longer pink, stirring and breaking up the sausage with a wooden spoon as it cooks.
Transfer the sausage to a cutting board and finely chop it. Let cool.
Preheat oven to 350°F. Brush the dirt off the mushrooms with a wet paper towel. Remove the stems from the mushrooms and place the caps in a 13x9 inch baking pan.
Crumble the Boursin cheese into a medium bowl. Add the chopped sausage and stir gently with a spatula to combine. Spoon heaping tablespoons of the Boursin and sausage mixture into each mushroom cup.
Bake the stuffed mushrooms for 25–28 minutes, or until the mushrooms are soft and the cheese is beginning to brown on the top. Garnish with chopped parsley, if desired, and serve while warm.
You can use spicy Italian sausage, if you prefer! I purchase it at the meat counter—it's called "bulk sausage" typically, and you can purchase exactly half a pound.If you can't find medium-sized mushrooms, you can use baby bella mushrooms or even the large portobello mushrooms. Adjust the cooking time accordingly: baby bellas will need less time, and large mushrooms will need more time. Store any leftovers in the refrigerator, and reheat for 7–8 minutes in a 350°F oven. I don't recommend freezing them—the mushrooms will get soggy when defrosted.