Cook the potatoes in a large saucepan of boiling, salted water for 15-20 minutes until tender and soft. Drain and leave to cool for a few minutes.
Mash with the chopped parsley, sour cream and seasoning until smooth. Cover and set to one side.
Preheat the grill to hot. Heat the butter in a casserole dish set over a moderate heat. Sweat the mushrooms for 6-7 minutes, stirring frequently. Drain in a colander to get rid of the excess water.
Spoon into a gratin dish and spread the mashed potato on top using a palette knife. Sprinkle the breadcrumbs and Gruyère on top and grill for a few minutes until the top is golden and crunchy. Remove from the grill and garnish with a sprig of parsley before serving.