To prepare the pierogi:

1. In a large skillet, heat the first tablespoon of vegetable oil over low-medium heat. Add the diced onion and sauté until caramelized, about 10-12 minutes. Remove from heat.
2. Meanwhile, chop the potatoes into 1-inch pieces and place in a saucepan filled with water. Boil potatoes until soft, approximately 10 minutes. Drain the potatoes. Mash the potatoes, then add the onions, vegan margarine, and salt and pepper to taste. Set aside the mixture to cool slightly.
3. In a large mixing bowl, sift together the flour and salt. Mix in the warm water and vegetable oil, whisking until a dough begins to form. Once you have dough, begin kneading for 5 minutes on a floured surface until the dough is very smooth and elastic.
4. Divide the kneaded dough into two and roll one portion out very thinly (approximately ?-inch, if you can). Using a 3-inch cookie cutter or the edge of a glass, cut out rounds of dough for each pierogi and set aside on a floured surface. Repeat with the remaining dough.
5. Take each round of dough and roll it out again, individually. Add 1 tablespoon of potato and onion filling in the center. Dip a brush or your finger in water and dab some water around the edge of the dough round, then fold in half. Seal the edge with your fingers, or by pressing down with a fork. Set aside. Repeat for each pierogi.
6. Bring a large pot of salted water to a boil over high heat. Boil 6-8 pierogi at a time, about 4 minutes each. Remove from the pot with a slotted spoon and continue until all pierogi are cooked.

To prepare the Brussels sprouts:

1. Preheat oven to 400°F.
2. Chop the Brussels sprouts into halves, then wash under cold water. Spread evenly on a prepared baking sheet and drizzle with tablespoon of vegetable oil, and salt and pepper to taste. Roast for 10-12 minutes, or until thoroughly cooked and crispy.

To assemble dish:

1. Add 1 tablespoon of vegetable oil to a large skillet over medium heat. Add some boiled pierogi, careful not to overcrowd the pan. Cook for 2-3 minutes on each side, until golden and crispy. Fry as many pierogi as you wish to eat, adding more vegetable oil if necessary. The remaining boiled pierogi may be stored in an airtight container in the refrigerator until ready to eat.
2. Keep the burner on and add the kielbasa to the pan. Cook for 2 minutes on each side, adding more vegetable oil if necessary.
3. If you do not want to fry the pierogi after boiling, simply set the boiled pierogi aside and keep warm.
4. Serve the pierogi with cooked kielbasa, roasted Brussels sprouts, sauerkraut, prepared horseradish to taste, and sliced green onions for garnish.