Method

• 1.
  Wash and peel potatoes, then cut into 3mm-thick slices (we used a mandoline).
• 2.
  Thinly slice white part of leek, and reserve green part for a stock or soup.
• 3.
  Heat a 30cm round cast-iron pan over medium heat. Add 50g butter, leek and 7 sage leaves. Cook for 5 minutes, stirring, until leek is soft but not coloured.
• 4.
  Add the sliced potatoes, arranging neatly to cover the surface of the pan. Add stock and remaining 50g butter. Cover surface of potato with a large, pre-scrunched piece of baking paper, allowing paper to come up sides of pan.
• 5.
  Increase heat to high and bring to the boil. Reduce heat to medium-low and cook for 40 minutes.
• 6.
  Remove baking paper and cook for 20 minutes or until the stock has reduced down and potatoes are soft but not breaking up. As the stock reduces, you’ll get a dark caramelised colour on the base of the pan (the caramelisation is important as it creates a depth of flavour).
• 7.
  Meanwhile, melt the extra 20g butter in a medium frypan over medium heat. Add extra sage sprigs and cook for 3-4 minutes until crispy. Set aside.
• 8.
  Serve gratin straight from the dish, topped with sage sprigs and very finely grated parmesan and seasoned with freshly ground black pepper.