Evenly sprinkle 1 cup of the kosher salt in a 2-quart baking dish. Evenly nestle the potatoes in the salt and place the rosemary sprigs on top. Cover the potatoes with the remaining salt until they are completely covered.
Bake in the preheated oven until the potatoes are tender and a knife inserted meets no resistance, 35-45 minutes.
Remove from oven and cool for 10 minutes.
While cooling, prepare the sauce. In a bowl, combine the Greek yogurt, buttermilk, mustard, rosemary, salt, and pepper. Whisk to combine and add more buttermilk if needed to thin the sauce. The sauce can be made the day before and chilled in an airtight container.
After the potatoes have cooled slightly, lift the potatoes from the salt. Wipe any excess salt from the potatoes.
Serve the potatoes on top of the salt in the baking dish alongside the prepared sauce.