Preheat the oven to 375°F (190°C). Lightly grease a deep baking dish.

In a large skillet over medium heat, melt the vegan butter. Add the torn seitan and cook for 3-4 minutes, stirring occasionally, until lightly browned. Add the minced garlic and carrots, cooking for another 2 minutes until fragrant.

Pour in the vegetable stock or water, bring to a simmer, and cook for 15-20 minutes until almost all liquid has evaporated and the seitan and carrots are tender. Remove from heat and set aside.

Using a food processor or box grater, shred the potatoes and onion. Transfer to a wire mesh strainer and squeeze and massage the onions and potatoes to release as much excess liquid as possible.

Place the shredded mixture in a large bowl along with the potato starch, matzo meal, salt, and pepper to the potatoes and mix until evenly combined.

Pour half of the potato mixture into the prepared baking dish, spreading it evenly.

Layer the cooked seitan mixture on top, then cover with the remaining potato mixture, smoothing the surface.

Melt the vegan butter in a saucepan, and drizzle it evenly over the top. Cover tightly with foil and bake for 50 minutes. Remove the foil and bake for another 20 minutes, until the top is golden and crisp. Let the yapchik cool and set up for 15 minutes before slicing. Serve warm with vegan sour cream or gravy and fresh herbs.