Preheat the oven to 220˚C/200˚C fan/gas mark 7.
Cut the potatoes into large chunks and place them into a large pan. Cover with cold water and bring to the boil. Simmer for 5 minutes, then drain well. In the meantime put the oil in a large roasting tin and place it in the oven to heat up.
Return the potatoes to the pan, add the polenta and flour, cover the pan with a lid and shake it very well to fluff up the potatoes and to coat them in the polenta and flour.
Carefully remove the roasting tin from the oven and add the potatoes, toss carefully to coat the potatoes with the oil, then add the garlic and rosemary. Season to taste.
Roast for 20 minutes, then turn the potatoes and roast for a further 20 minutes, until they are crisp and golden. Serve with the extra rosemary and garlic cloves to garnish.