Place the potatoes in a large stock pot and cover with cold water by at least one inch, then add the salt.
Place the stock pot on the stove, turn the heat to high and bring to a boil, then allow to briskly boil until the potatoes crack apart when pressed with a fork, about 15-20 minutes, then drain in a colander.
Return the pot to the stove and turn the heat to medium. Add 1 tbsp of the vegan butter and allow it to melt, then add the greens.
Cook until the greens are soft, about 4 minutes, then add the green onions and cook for another minute, until the onions begin to soften up.
Turn off the heat, add the potatoes back to the pot, then add the remaining 4 tbsp butter and the oat milk and use a potato masher or fork to mash the potatoes together with the greens until the potatoes are the consistency of mashed potatoes and the vegetables are evenly distributed.
You may need to add more oat milk and vegan butter, depending on what consistency you prefer.
Season with salt and pepper to taste, add more vegan butter, and serve.
