Wash and peel potatoes, then cut into 3mm-thick slices (we used a mandoline).

Thinly slice white part of leek, and reserve green part for a stock or soup.

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.

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.

Increase heat to high and bring to the boil. Reduce heat to medium-low and cook for 40 minutes.

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).

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.

Serve gratin straight from the dish, topped with sage sprigs and very finely grated parmesan and seasoned with freshly ground black pepper.