1. Place the potatoes into a large pot and cover with water. Bring to a boil over high heat, then reduce the heat to medium-low, cover, and simmer until just tender, 10–15 minutes. Be careful not to overcook the potatoes and create a mushy texture.
2. Sauté the cabbage, leeks, and onion in the vegetable stock until tender, then strain to remove moisture.
3. In a bowl, mash the potatoes with the milk, garlic, nutritional yeast, and chives.
4. Fold in the cabbage and leek mixture and season with salt and pepper.