Put whole potatoes in a large pan. Cover with water. Bring to a boil. Cook for 10 to 15 minutes, until tender. Drain, cool, peel, grate into a bowl.

Heat 2 tbsp of butter in iron pan. Add onion and bacon. Cook for 5 to 6 minutes, until onions soften. Tip mixture into potatoes. Season; mix well.

Heat half of remaining butter in another fry pan. Add potato mixture. Press down to form a large pancake. Cook 10 minutes, adding extra butter around edges if needed; shake pan occasionally.

Cover pan with a plate and flip over. Add more butter, then slide roesti back to cook other side. Add more butter as needed. Cook for 7 to 10 minutes or until golden. Take off heat; keep warm.

Heat last of the butter in a pan. Add mushrooms. Sauté, stirring, for 3 to 5 minutes. Season; stir in parsley.

Sprinkle Canadian Gruyère cheese on top. Place under broiler for 1 to 2 minutes.

Top with mushrooms. Serve.