Place a 10-inch cast iron skillet (or other oven-safe skillet) in the oven and turn on the heat to 375°F. Peel the potatoes and shred them using the coarsest side of a box grater, then do the same with the onion half (this can also be done in a food processor using the shredder attachment). Transfer the shredded potatoes and onion to a large, clean dish towel and squeeze to remove as much liquid as possible. The shredded potatoes may begin to oxidize and turn a rusty red color—this is normal and won’t affect the final product. Transfer the mixture to a large mixing bowl and add the paprika, salt, turmeric, cumin, cinnamon, and several grinds of freshly cracked black pepper. Mix until well combined. Remove the skillet from the preheated oven and add 1 tablespoon of the oil as well as the butter. Gently swirl the pan until the butter is melted, then add the potato mixture and use a spoon or spatula to spread it into an even layer. Brush the remaining oil over the top of the potato mixture, then return the pan to the oven and bake for about 1 hour, or until very golden brown and crispy. Meanwhile, make the sauce. Add the Greek yogurt, mayonnaise, pepperoncini juice, garlic, and salt to a small mixing bowl and stir until well combined. When the rösti is finished, transfer it to a serving plate. Cut into wedges and serve topped with poached eggs, white sauce, sliced pepperoncini, parsley, and sriracha, if desired. Store leftovers in an airtight container in the refrigerator for up to 3 days. Enjoy leftovers cold or reheat in a 375°F oven for 5 to 10 minutes, or until warmed through and re-crisped.