Place the potatoes in a large pot of water and bring to a boil. Reduce heat to medium, and cook until tender, about 20 minutes.
Put the thinly sliced cabbage in a separate pot of water and bring to a boil. Reduce heat and cook until cabbage turns a darker color, and is tender, about 5 or 6 minutes. Drain.
When the potatoes are done cooking, drain the water from the pan.
Add the vegan buttery spread, and mash the potatoes with a potato masher.
Add the non-dairy milk and stir, adding more if the potatoes seem to dry.
Add the cooked and drained cabbage, and the salt and pepper. Stir.
Stir in the diced green onions, reserving a few for garnish if desired.