In a large bowl, place the sour cream, milk, beaten egg and salt, and whisk to combine well. Add the flour and xanthan gum in three parts, mixing well to combine after each addition.

The dough will come together and be relatively smooth but still a bit sticky. Turn it out onto a lightly floured surface, and, with floured hands, knead the dough until it becomes easier to handle (it will not be completely smooth).

Cover the dough loosely with plastic wrap and set it aside to rest.

Bring a large pot of salted water to a rolling boil, add the potatoes and reduce the boil to a simmer. Boil the potatoes, covered, until they are just fork tender (about 15 minutes).

Drain the water from the pot of potatoes and cover the pot with a tea towel. Allow the potatoes to steam beneath the towel until the are softened (about another 10 minutes).

While the potatoes are cooking, place the diced onion and clarified butter in a small skillet and cook, stirring frequently, until the onions are translucent (about 6 minutes).

Once the potatoes are softened, peel the skin off (it should come off easily) and mash the potatoes in the pot. Add the cooked onions, then the sour cream, shredded cheese and the salt and pepper to taste. Set the filling aside.

On a lightly floured surface, roll out the dough into a round about 1/4 inch thick, and moving the dough frequently and dusting lightly with flour to prevent sticking as you roll.

Cut out rounds 4 inches in diameter from the dough. Gather and reroll scraps. Roll each round into an oval that is closer to 1/8 inch thick (the thickness of a nickel).

Paint the edges of each oval with the beaten egg, then place about 1 tablespoon of filling in the center of each.

Fold the dough over on itself, matching the edges, and pinch the edges together to form a tight seal.

In a large pot of salted, boiling water, place the pierogies, about 6 at a time, until they float to the surface of the water (about 3 minutes).

Place the boiled pierogies on a paper towel to drain, then blot them dry.