2 large leeks, topped and tailed and halved lengthways
675g (1½lb) potatoes, peeled
25g (1oz) plain flour
5ml (1 tsp) freshly grated nutmeg
5ml (1 tsp) salt
2.5ml (½ tsp) freshly ground black pepper
50g (2oz) butter, cut into small pieces
450ml (15fl oz) milk
50g (2oz) Gruyère cheese