Preheat oven to 425°F.

Fill a large bowl with cold water and, as you peel potatoes, place in cold water to prevent browning.

Cut potatoes lengthwise into halves or quarters so they fit into food processor feed tube. Process potatoes and onions using the blade that creates thin, shoestring-like strips.

Place ¼ cup evoo in a cast iron pan and either heat in the oven or over medium heat on the stovetop.

Transfer potatoes and onions to a large bowl, add potato starch, salt and pepper, mix well. Remove any large pieces of potatoes or onions that weren't processed properly.

Remove hot pan from the oven and carefully pour potato mixture into the pan. Drizzle with remaining ¼ evoo.

Bake at 425°F for 1 hour. If the edges are still pale, cook for 20 minutes longer until the top looks crunchy and sides look golden and browned. Loosen edges with a knife and serve straight from the pan.