Preheat oven to 250°C/500°F/Gas Mark 7. Mix together mashed potato and water. Add yeast, stir well and stand for 20 minutes. Tip into a large bowl and add half the flour, mix well then add polenta, salt and half the remaining flour.
Knead, working into a moist but manageable dough, adding more flour if necessary. Add half the rosemary, knead again, then cover the bowl with plastic wrap and leave in a warm place for 40 minutes.
Knock dough back and tip onto a lightly oiled baking tray. Press or roll out to a ½ cm (¼ in) thickness. Sprinkle with finely sliced red onion rings and remaining rosemary. Crumble over feta, drizzle with half the oil and leave to rise in a warm place for 20 minutes. Bake on a low shelf for 20 minutes or until base is crisp. Move to top of oven and cook a further 10 minutes. Remove from oven and drizzle with remaining oil. Best eaten straight from the oven, but it makes a good picnic bread too.