5. Meanwhile, heat the remaining butter in a small, heavy-based frying pan. Add the thyme and onions and fry gently for 10 minutes until the onion is softened.
066. Preheat the oven to 375°F
077. Slice the fried potato rosti onto a baking sheet. Spread the fried onion over the rosti and layer with the Parma ham, then top with the Emmenthal.
088. Bake the rosti for 10 minutes until the cheese has melted. Serve warm from the oven.