Place the freshly fed sourdough starter, water, salt, olive oil, and sugar in a stand mixer. Mix well until the starter is fully dissolved.

Add the flour and mix until a smooth, elastic dough forms and wraps around the dough hook.

Remove the dough from the mixer and knead it briefly on a lightly floured surface. Shape it into a rough rectangle and place it in a greased baking pan. Drizzle a little olive oil from the pan over the dough and let it rest in the turned-off oven for several hours. For best results, let it rise overnight.

With your fingers dipped in a mixture of oil and water, gently stretch the dough to fill the pan and make the classic dimples. Brush with extra olive oil if you like—a good focaccia is measured by how deliciously oily it is. Let it rise for another couple of hours in the turned-off oven.

Meanwhile, peel the potatoes and slice them thinly. If you use a vegetable peeler and small potatoes, you'll get even slices. Place them in a bowl, toss with olive oil, rosemary, and a little salt, and let them marinate until you're ready to top the focaccia.

Arrange the potato slices in overlapping rows on the focaccia. Bake at 430°F (220°C) for 25 minutes on the lower rack of the oven. Serve hot!