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Cook the potatoes in boiling water for 15 mins, then drain and leave to cool completely. Meanwhile, boil the kettle.
Pour 175ml boiling water and 100ml cold water into a large bowl. Sprinkle over the yeast, 1 tsp salt and 2 tbsp olive oil. Mix well, then stir in the flour, adding a splash more water if the mixture feels too dry.
Turn out the dough onto a clean work surface and knead for 15 mins until smooth and elastic (dust the surface with a little flour if the dough is sticky), then shape into a ball. Grease the inside of a large bowl with olive oil, then add the dough to the bowl and turn a few times to coat in the oil. Cover with cling film and leave in a warm place for 1 hr or until doubled in size.
Grease a large roasting tin (about 26cm x 35cm) with olive oil and dust with a little flour. Heat oven to 200C/180C fan/gas 6. Carefully stretch out the dough to fit the tin, pressing it into the corners. Cut the cooked potatoes into wedges and tuck them into the dough at regular intervals, along with sprigs of rosemary, and scatter a few sprigs over the top. Cover the tin with cling film and leave to prove for another 45 mins. Remove the cling film, pour over 3 tbsp olive oil and sprinkle over a little salt. Bake in the oven for 30-35 mins or until the bread is golden and the potatoes are browning lightly.
When the bread is cooked, remove it from the oven and quickly drizzle over another 3 tbsp olive oil while it’s still hot. Leave to cool for 10 mins, then turn out onto a board, slice and serve.