Put the yeast, sugar and 330ml warm water in a large bowl and stir. Let stand for 5 minutes until foamy. Stir in half of the oil and 1 teaspoon of the salt.
Add the flour, a little at a time, stirring until the dough forms a ball. Knead on a floured work surface until smooth and elastic. Add extra flour to keep the dough from sticking. Put the dough in a lightly oiled large bowl and cover with oiled clingfilm. Leave in a warm place to rise until doubled in size - between 1-2 hours.
Meanwhile, preheat the oven to 220°C, fan 200°C, gas 7. Punch the dough to knock out the air and gently knead in the mashed potatoes, being careful not to overwork. Stretch or roll out the dough and place on a floured baking tray. Poke holes in the loaf and fill each with a sprig of rosemary. Drizzle over the remaining oil and salt. Bake for 25-30 minutes until golden.