First make the starter. Combine all the ingredients in a screw-top jar, beating until they form a smooth paste. Screw on the lid and set aside in a warm place for between 3 and 4 days.

Sift the flour, sugar and salt into a bowl. Make a well in the centre and pour in the starter, water and oil. Using your fingers, gradually draw the flour into the liquids Until it is incorporated and the dough comes away from the sides of the bowl.

Turn out on to a floured board and knead for 5 minutes. The dough should be elastic and smooth. Shape the dough into a ball and return it to a clean greased bowl. Cover and put in a warm, draught-free place for 2 hours, or until it has risen slightly.

Preheat the oven to fairly hot 190°C (Gas Mark 5, 375 °F). Grease a large baking sheet with the butter.

Turn the dough on to a floured board and knead for a further 10 minutes. Cut into two pieces and shape each one into a round, about 15 cm (6 in) in diameter. Make a deep cross on the top of each round. Put the rounds, well spaced apart, on the baking sheet and put the sheet into the oven. Bake for 1 to 1 1/4 hours.

Remove the bread from the oven and rap the undersides with your knuckles. If the bread sounds hollow, like a drum, it is cooked. If it does not, reduce the oven to warm 170°C (Gas Mark 3, 325°F), return the loaves and bake for a further 5 to 10 minutes. Cool on a wire rack before serving.