For the filling: Boil the potatoes in salted water until soft. Rinse and drain well.
While still hot, mash the potatoes with the cream cheese, American cheese, and mozzarella cheese. Taste and add salt and pepper. Refrigerate until firm which will make it easier to fill the pierogi. Any extra filling can be served in place of regular mashed potatoes.
For the dough: In a large bowl, combine the flour blend, chickpea flour, xanthan gum, and salt. Mound the flour mixture and make a well in the center. Drop the eggs into the well and cut into the flour with a knife to break the yolks. Add the water and knead until firm. The dough will be sticky, but do not add additional flour.
Place the dough on a work surface and let rest for 10 minutes covered with plastic wrap. Divide the dough in half. Between sheets of plastic wrap that have been sprayed with vegetable oil spray, roll each half into a thin (1/16 of an inch) circle. Cut the circles with a 3- to 4-inch biscuit cutter. Roll out the excess dough and cut until all the dough has been used.
Place a small spoonful of the filling a little to one side of each round of dough. Do not overfill or the pierogi will be difficult to seal. Fold over and pinch the edges together firmly, using oiled plastic wrap. Use the tines of a fork to form lines along the edges, if desired. If they are not sealing well, moisten the edges with water. Cover with plastic wrap.
Bring a large skillet of water to a boil and reduce to a simmer. Add salt and olive oil. Drop the pierogi into the water. Do not crowd the pan or they will stick together. Cook for 3 to 5 minutes, stirring gently to make sure they do not stick to the bottom of the pan. They will rise to the top as they cook. Lift out of the water carefully with a slotted spoon.
Arrange in a single layer on a large platter. Do not crowd or pile the pierogi. The uncooked will stick and the cooked will lose their shape and lightness.
To serve: In a medium skillet, melt 1 tablespoon of the butter over medium heat and sauté the onion until nicely caramelized, about 10 minutes. Remove from the pan. Heat the remaining butter and sauté a few pierogi at a time. Serve pierogi with the caramelized onions, sour cream, and sauerkraut or a mixture of these.
Boiled pierogi can be frozen. Cool completely and wrap in a single layer in wax paper and then place in a zip-top bag to freeze. Reheat by sautéing or boiling briefly