1.Combine yeast, sugar and the water in small bowl; cover, stand in warm place about 10 minutes or until frothy.
2.Sift flour and salt into large bowl. Stir in yeast mixture and olive oil; mix to a soft dough. Bring dough together with hands; add a little extra water if necessary.
3.Knead dough on floured surface about 10 minutes or until smooth and elastic. Place dough in oiled large bowl; cover, stand in warm place about 1 hour or until doubled in size.
4.Preheat oven to 240°C/220°C fan-forced.
5.Punch dough down with your fist, then knead on floured surface until smooth. Divide dough in half. Roll each half to a 20cm x 35cm rectangle, then place on oiled rectangular trays. Sprinkle with polenta; prick bases with a fork.
6.Layer potato, overlapping slightly, over top of pizza. Sprinkle with rosemary, drizzle with combined garlic and extra oil.
7.Bake pizza about 15 minutes or until base and potato are browned and crisp.