To make the dough, stir the flour and salt together in a medium mixing bowl. Stir in the water and butter to form a dough. Transfer the dough to a lightly floured surface and knead for about 6 minutes, until smooth and elastic. Transfer the dough to a lightly oiled bowl and cover. Allow it to rest for 1 hour.

While the dough rests, caramelize the onions. Place the onions and oil into a medium skillet and stir a few times to coat the onions with oil. Set the skillet over medium-low heat. Allow to cook, stirring occasionally, until the onions become very soft and light brown, about 45 minutes.

While the dough rests and the onions caramelize, place the potato into a medium pot and cover with water. Place the pot over high heat and bring the water to a boil. Allow to cook until the potato is fork tender, about 15 minutes. Don't overcook it.

Drain the potato into a colander and return it to the pot. Mash the potatoes with a potato masher and stir in the milk, nutritional yeast flakes, white wine vinegar, garlic and salt. When the onions are finished cooking, stir them into the potato mixture.

When the dough has sat for 1 hour, split it into 2 halves. Transfer one of halves to a lightly floured surface and roll to a square of about 14 x 14 inches. Cut 3-inch circles using a drinking glass or pastry cutter. Reroll the dough as needed, and repeat with the other half. You should get about 24 circles.

Place about 1 tablespoon of the potato mixture onto each dough round, then fold in half and seal tightly to form a pierogie. Apply a bit of water if you have trouble sealing the dough.

Bring a large pot of water to a boil and add the pierogies (work in batches if it gets too crowded). Cook the pierogies for 3-4 minutes, until they float, removing each one with a slotted spoon and transferring to a plate as it begins to float.

To fry the pierogies, melt the butter in a large skillet, then add as many pierogies as will fit without crowding. Cook for about 4 minutes on each side, until golden and crisp. Transfer the pierogies to a paper towel-lined plate. Add butter to the skillet between batches as needed.

Serve with sauerkraut, dill, chives, applesauce, and/or vegan yogurt or cashew cream.