Place the potatoes in a large pot, cover them with water, and bring to a boil. Cook until the potatoes are tender. Drain the potatoes, allow them to cool completely in the refrigerator, then cut them into 1/4-inch slices. (You can also microwave or bake in the oven - your choice! And I usually don't allow them to cool all the way - just enough to handle.).
Heat the oil in a large nonstick wok or skillet on medium-high heat. Add the potatoes, onion, mushrooms, the white part of the green onions, and seasoning or salt and pepper. Cook, tossing occasionally, until the potatoes are crisp and brown—approximately 10 minutes.
Top the potatoes with the green part of the green onions, the avocado, nondairy sour cream, and hot sauce and serve immediately.