Whisk the flour and salt together in a large bowl. Pour in the warm water and the oil, then mix with a fork until a shaggy dough forms. Turn it out onto a lightly floured surface and knead for five to seven minutes, until the dough is smooth and only slightly tacky. If it sticks to your hands, work in flour a tablespoon at a time.

Cover the dough with a clean towel or plastic wrap and let it rest at room temperature for thirty minutes. Resting relaxes the gluten, which is what lets you roll the circles thin without them shrinking back. Make the filling while it rests.

Put the potatoes in a pot, cover them with cold water by an inch, and add a generous pinch of salt. Bring to a boil and cook for twelve to fifteen minutes, until a knife slides into a chunk with no resistance. Drain well and let the potatoes steam dry in the colander for a minute.

While the potatoes cook, warm the olive oil in a wide skillet over medium-low heat. Add the onions and cook, stirring now and then, for twenty to twenty-five minutes, until they're soft, deep golden, and jammy. Stir in the garlic for the last minute. Scoop out about a third of the onions and set them aside for serving.

Mash the drained potatoes with the caramelized onions, vegan butter, nutritional yeast, salt, and pepper until smooth. Spread the filling out in a bowl and chill it for at least twenty minutes, or up to two days, so it firms up and won't steam the seams open.

Divide the dough in half and keep the half you're not rolling covered. On a floured surface, roll one half into a sheet about 1/8 inch (3 mm) thick. Cut out 3-inch circles with a cookie cutter or the rim of a glass, then gather the scraps, re-roll them, and cut again.

Set two teaspoons of the chilled filling in the center of each circle. Fold the dough over into a half-moon, press out the air as you go, and pinch the edges together. Crimp the seam with the tines of a fork, and keep the assembled pierogi under a towel so they don't dry out.

Bring a large pot of well-salted water to a gentle simmer, not a rolling boil. Lower in a batch of pierogi without crowding the pot and cook them for about five minutes, until they float, then let them cook for two more minutes. Lift them out with a slotted spoon and set them on a plate while you cook the rest.

Melt a tablespoon of vegan butter in a wide skillet over medium-high heat. Fry the boiled pierogi in a single layer for two to three minutes per side, until they're golden and crisp in spots. Work in batches, adding more vegan butter as needed, and serve them hot with the reserved onions, vegan sour cream, and fresh herbs.