Preheat oven to 350° F. Grease an 8x8 inch (or equivalent) casserole dish.
Fill a large pot with cold water and about 1/2 teaspoon of salt.
Peel potatoes and cut into 2-inch chunks (+/-)
Place potatoes into the cold water in the pot and bring to a boil.
Continue to cook until potatoes are knife tender, but not too mushy. This time varies depending upon the size of the potato pieces, but should take approximately 20 minutes or so. Keep your eye on them and test them frequently so they don’t get too mushy.
Once the potatoes are tender, drain them in a colander and cool enough to handle.
While the potatoes are still warm, mash them using the food grinder on your KitchenAid, a food processor or a good hand masher and move them into a large mixing bowl.
Immediately add the goat cheese to the mashed potatoes while they are still warm and mix well to get the goat cheese well-integrated.
Add the mashed celery root (directions below), buttermilk, minced herbs, salt and white pepper and stir well.
Turn the potato mixture into the prepared casserole dish. (Note- this whole process can be done a day ahead of time and stored in the refrigerator until ready to serve.)
Cover the casserole dish with foil and bake for 20 minutes, or until heated through. Remove foil and continue to bake for another 10 minutes to get a little color on the top.