Makes about 400 g (14 oz)
2 medium potatoes, peeled and cut into 2 cm (3/4 in) dice
water
plain bread flour
Cook the potatoes in 475 ml (16 fl oz) water until soft. Mash lightly with a fork into the water and leave to cool. Add 250 g (8 oz) of the flour and mix very thoroughly until you have a smooth batter. Put into a sterile glass jar or jug and seal tightly with clingfilm. Leave in a warm dark place for two days. (An airing cupboard is good.)
Stir the starter which will have begun to ferment and expand. Re-cover and leave for another day.
Uncover the starter, add 100g (4 oz) flour and 4 tablespoons warm water. Stir to mix. Cover again and put back in the warm place for two more days.
Uncover the starter and put into a warm bowl. Add a further 200 g (7 oz) flour and 4 tablespoons warm water. Mix together until the flour is incorporated, and then leave covered with a cloth to double in volume. It takes about 6 hours before the sourdough is ready for use. Store in the fridge, tightly covered with clingfilm.