Let's start with the potatoes: cook them (do not throw away the cooking water) and mash them in a bowl. Add the yeast to the warm potato cooking water and let it dissolve. Now pour the water with the yeast into the potatoes and sift the flour into the same bowl. If you have a planetary mixer use it, otherwise mix with a spatula. Add salt and knead the dough until the surface becomes smooth. Take a round baking tray and put the baking paper. Then place the dough inside, spreading it out and trying to make it homogeneous in thickness. Now pour in some oil. Cut the cherry tomatoes and place them on the focaccia. Cover the focaccia with a torch and let it rise for 1 / 1:30 h in an oven turned off or away from cold currents (avoid hot places). Then add more oil and bake for 40 min in a 220° oven.