Heat the butter in the cooker. Saute the garlic, celery, and scallions for a m inute or two. Stir in the onions and potatoes, tossing to coat with the butter . Add the stock and 1/4 cup parsley. Lock the lid in place and over high heat bring to high pressure. Adjust heat to maintain high pressure, and cook for 5 minutes. Let the pressure drop naturally or use a quick release method. REmo ve the lid, tilting it away from you to allow any excess steam to escape. Add the remaining parsley and milk (if using). Over low heat, gradually stir in th e grated cheese, simmering until the cheese is melted and the soup is hot. Add salt and pepper to taste before serving.