These mashed potatoes are as easy as (sweet potato) pie and pretty versatile. You can cook the sweet potatoes ahead of time and then mash them before you serving.
These can also be made vegan and dairy-free by substituting the butter and milk for a vegetable-based margarine spread like Earth Balance and vegetable broth.
Either cook the sweet potatoes by putting them in a microwave-safe dish with enough water to cover the bottom, poking a few holes with a fork and microwaving on high for 10 minutes, or boiling for 20 minutes in a 1/2 quart of water. I leave the skins on but it means that your mashed sweet potatoes will not be completely smooth.
While the potatoes are cooking, heat the grapeseed oil in a skillet. Turn to low heat. Add the walnuts and toast for about 5 minutes. Transfer to a plate or paper towel and add the salt and sugar.
On very low heat, mash the sweet potatoes in a large sauce pan (or you can use a ricer and then add them to the pan). Add the milk, butter and spices until blended. Take a fork and whip the mashed sweet potatoes until they are pretty smooth.
Scoop onto plates and sprinkle the toasted walnuts on top.