Method

• 1.
  Preheat oven to 180°C. Heat oil in a large pan over medium-low heat and cook onion, stirring, for 15 minutes or until soft.
• 2.
  Stir in horseradish, then set aside to cool. Rinse potato under running water to remove starch. Spread on a clean tea towel and pat dry with paper towel.
• 3.
  Grease the base of a 20cm ovenproof frypan with the ghee. Arrange the potato over base in overlapping circles, brushing each layer with ghee and seasoning as you go. When you have used half the potatoes, sprinkle over onions and Gruyere.
• 4.
  Continue layering potato. Bake on the bottom shelf of the oven for 1 hour or until tender. Run a knife around the pan, then press a plate against the potatoes and tilt the pan over a bowl to catch the excess ghee.
• 5.
  Slide the potato cake onto a board and serve crisp-side up.
• 6.
  Drizzle tart with truffle oil, if using, and serve with a ploughman’s platter