1) Peel and grate both potato types and rinse them really well under cold water. Use a bunch of paper towels or a sturdy kitchen towel to dry them really well.
2) Preheat oven to 375 degrees F. In a large (12 inch) cast iron skillet, add 2 tablespoons of oil and heat over medium heat.
3) Toss grated potatoes with chopped scallion whites, salt and pepper, and add to cast iron pan. Spread out into an even layer. Cook for 6-7 minutes until it is starting to brown around the edges.
4) Carefully loosen the potatoes from the bottom and slide them onto a plate. Then (with confidence) flip them back into the pan.
5) Drizzle hash browns with another tablespoon or two of oil and place in the oven for 25-30 minutes until the potatoes are well-browned.
6) Remove from the oven, garnish with scallion greens and coarse salt, slice into quarters and serve immediately!