- Heat a deep-fryer to 175C. Shape the potato into a block, cut 2mm slices, then matchsticks, and deep-fry, stirring occasionally until golden brown. Tip on to absorbent paper to drain, season with salt.
- In a mixing bowl, cream the butter until pale, blend in the garlic, the juice from the quarter lemon and parsley. In another bowl, blend the yoghurt with the horseradish, the juice from the whole lemon and pepper. Preheat a cast-iron grilling pan or the barbecue's solid top.
- Smear the butter over the steak, place this side down on to the grill, fry for three minutes, then turn, smear with more butter and fry for three minutes. Place steaks on to a cooling rack for four minutes.
- Place a mound of horseradish sauce and straw potato on warmed plates, garnish with watercress.