Breakfast Sausage Stuffed Mushrooms with Cornflakes

Name: Breakfast Sausage Stuffed Mushrooms with Cornflakes
Ingredients: 24 large white or cremini mushrooms, wiped clean, stems finely chopped for the filling; 2 tablespoons extra-virgin olive oil; Kosher salt; 4 tablespoons (1/2 stick) unsalted butter; 1/2 cup finely chopped celery; 1/2 cup finely chopped onion; Kosher salt and freshly ground black pepper; 1/2 cup crumbled cooked breakfast sausage; 1/2 cup grated mozzarella; 2 tablespoons finely chopped fresh chives; 2 tablespoons finely chopped fresh flat-leaf parsley; 1 cup crushed cornflakes