Preheat grill to highest setting.
Heat a saucepan of water until boiling and then par-boil the diced potato for 4-5 minutes.
Drain and stir through wholegrain mustard and rosemary with a fork.
Place a ring mould on a baking sheet and fill with the potato mixture, pressing down.
Top with crumbled Stilton cheese and place under the grill for 2-3 minutes, until bubbly. Serve.