Shred the potatoes with a box grater or Kitchen Aid food processor.
Season the shredded potatoes with kosher salt and thyme leaves, and mix to combine.
Place the potatoes in a kitchen towel. Twist the top of the towel to apply pressure, the potatoes will start releasing liquid. Try to remove as much water as possible.
Heat the cast iron pan over medium heat. Add half of the oil to the pan. Gently place the potatoes in the pan, and gently press the potatoes down. The oil should be bubbling along the edges of the potatoes.
Cook the potatoes on top of the stove for 5 minutes.
Place the cast iron pan on a baking sheet, the oil can bubble over the edge while cooking in the oven.
Check the potatoes after cooking for 20 minutes, there needs to be oil bubbling up along the edge, if the potato is dry add the remaining oil.
After cooking for 35 minutes, gently flip the crispy potato rosti. Continue to cook for another 25 minutes.
The Rosti needs to be a deep golden brown in color on both sides.
Remove the rosti from the pan, and place it on a drain rack.