Bring a medium saucepan of water to a boil over high heat. Add the potatoes and boil until soft, 12 to 15 minutes, then drain.
Meanwhile, in a medium skillet, melt the butter over medium heat. Sauté the onion until translucent, about 4 minutes.
With a hand mixer or stand mixer, combine the potatoes, cheese, and onions. Mix well to combine. Season with the salt and pepper. Allow the potato-cheese mixture to cool completely in the fridge.
Divide the dough into 20 equal portions. Use additional tapioca starch, as needed, to keep the dough from sticking to the counter or your hands.
Take a piece of dough and shape it with your fingers into a 3-inch round circle. Fill with 2 tablespoons of the chilled potato filling. Fold the dough in half around the filling and gently stretch so the edges meet. Pinch and
press the edges together to seal.
Bring a large-size pot of salted water to a boil. Working in small batches, boil the pierogi for 8 minutes. Scoop out with a slotted spoon,
arrange in a single layer on a cookie sheet (with sides), and drizzle with the melted butter. Turn so all are coated well to ensure they do not stick to each other.
Eat immediately or lightly brown in a skillet over medium heat (using additional butter, as needed). Top with sautéed onions and sour cream, if desired.
After the pierogi have been boiled, they can be individually frozen, once cooled, on a baking sheet or plate in a single layer. Once frozen, transfer them to a zip-top plastic bag for up to 6 months. Cook from frozen for best results; do not thaw prior to cooking.