- Heat the oil in a frying pan. Lightly fry the leek and onion until coloured.
- Add the lamb and stock. Heat through until piping hot. Transfer the mixture to an over proof dish.
- Overlap the sliced potatoes around the edge of the dish.
To make the rarebit:
- Combine the cheese, mustard and ale to a thick paste.
- Spread the paste over the potatoes. Place under a hot grill for 10 minutes, until golden brown.