Add the sliced potatoes to a saucepan of boiling water, bring back to the boil and boil for 1 minute. Drain the potatoes and place in a bowl with the sliced onion and rosemary. Drizzle the olive oil over and carefully mix so as to not break the potato slices; leave to stand.
Add 125g flour to a bowl with all the sugar, salt and yeast and mix well. Warm the water and oil, then add to the flour and yeast mixture carefully mixing with a wooden spoon or mixer. Beat for 2 minutes whilst adding 200g flour until the mixture pulls away from the sides of the bowl. Add the remaining flour while kneading in. Cover and allow to stand for 5 minutes.
Grease a baking tray, or line with greased baking parchment. Form the dough into a ball and place on the tray. Roll out to a 15cm wide circle. Cover with greased cling film, a cloth towel and leave in a warm place for 30 minutes.
Remove the cloth and cling film and make holes using the end of a handle on a wooden spoon. Place the potato mixture in a layer across the dough and pour the oil from the bowl over.
Preheat the oven to 230 C / Gas 8.
Bake in the oven for 25 to 30 minutes until golden brown and risen. Remove from the oven and allow to cool on a cooling rack. Eat and enjoy!