Begin the starter, the night before you want to make the bread.
Combine ingredients for the starter in a bowl or glass jar and mix well to combine.
Cover with a damp cloth and leave for at least 3 hours at room temp or overnight.
In the bowl of a mixer, combine the 1 ½ cups PPP flour xanthan gum, sea salt and baking powder using a paddle attachment. Add the starter, which should be bubbling and the water and olive oil and mix for 2 minutes.
Using the remaining ¼ cup of flour roll the dough out into a rectangle approx. 40cm x 20cm then transfer to a piece of baking paper in a bench brush with the olive oil and sprinkle with the sea salt. Cover with a tray and allow to rise somewhere warm for 40 minutes to 1 hour.
Preheat your oven to 210C. and place a baking tray in the oven for 30 at least 30 minutes.
Thinly slice the potato on a mandolin or with a knife to approx. 2mm thick. Toss this with the onion, garlic, olive oil, rosemary, some salt, pepper and evenly lay over the focaccia, just overlapping.
Remove the hot tray from the oven, lift the focaccia, on baking paper, onto the tray and place back in the oven for 20 minutes. Remove the focaccia, add the cheese and return to the oven for 5-7 more minutes to melt the cheese and finish cooking