500 gm(about 2) waxy potatoes (such as Dutch cream, pinkeye or désirée), thinly sliced with a mandolin550 gmmozzarella, sliced300 gmGruyère, coarsely grated8sage leaves, coarsely tornPizza dough15 gmdried yeast400 gm"00" plain flour, plus extra for dusting2 tbspextra-virgin olive oil, for brushing
For pizza dough, combine yeast and 40ml lukewarm water in a small bowl and stir until dissolved, add 2 tbsp flour and stir until combined. Cover and stand in a warm place for 30 minutes or until foamy. Place remaining flour in a large bowl, make a well in the centre and add yeast mixture, 1½ tsp sea salt and 200ml lukewarm water. Mix ingredients together then knead on a lightly floured work surface until dough is smooth and elastic (8-10 minutes). Divide dough into 4 and place on a lightly greased oven tray, brush with olive oil, cover with a tea towel and stand in a warm place until doubled in size (2 hours).
Place a pizza stone on the middle rack of an oven and preheat on highest heat. Lightly dust a work surface with flour and, using fingertips, press out dough to about 20cm in diameter. Arrange one-quarter of the mozzarella evenly over pizza base, arrange potatoes on top then scatter over one-quarter of the Gruyère and sage. Using a thin tray, slide pizza onto preheated pizza stone and cook until golden (4-5 minutes). Repeat with remaining dough and toppings. Serve immediately.