In a large pot, place the cubed potatoes, cover with water, and bring to a boil. Reduce heat to low and simmer for 20 minutes or until potatoes are tender.
When the potatoes are tender, drain them, reserving the cooking liquid for making soup or for use in another dish, and return the cooked potatoes to the pot.
Add nondairy milk. Using a potato masher, mash the potatoes, making the mixture as smooth or chunky as desired.
Add the vegan buttery spread, salt, and pepper, and stir well to combine.
Keep warm while making gravy.
For the gravy: In a medium saucepan, place flour, and cook over low heat, while stirring constantly, until lightly browned and fragrant, about 2 to 3 minutes. Transfer browned flour to a small bowl and set aside.
In the same saucepan, sauté the onion in olive oil, over low heat, for 3-5 minutes or until soft. Add garlic and sauté an additional 2 minutes.
Add vegetable stock, nutritional yeast flakes, tamari, sage, thyme, salt, and black pepper to browned flour, and whisk well to combine.
Add wet ingredients to sautéed onion mixture, whisk well to combine, and continue to cook mixture, while whisking constantly, until thickened.
Taste and adjust seasonings, as desired. Serve over the mashed potatoes or any of your favorite vegetables, biscuits, or main dishes, or use to make sauces for casseroles.