I was trying to duplicate a side dish I had at a restaurant that's since gone out of business... I didn't do it, but what I came up with came out pretty good!
1/2 of a rutabaga or turnip
2 small or 1 large sweet potato
6 average-sized (4" long) potatoes
1 tbsp safe margarine
1/2 of a small roasted onion - optional, but you'll want some onion powder if you don't use it
Plain soy milk
Salt & white pepper to taste
Peel all the veggies and cut into chunks. Put the turnips into a large pot of water, bring to a boil, and cook until almost fork-tender. Add the sweet potatoes and potatoes and cook until everything is very soft. Drain.
Add to the bowl of a food processor along with the margarine and roasted onion. Process until everything is pureed. SLOWLY add soy milk, a little at a time, until desired consistency is reached - you'll only need a couple tablespoons. Season to taste with salt and white pepper.
You can also use an electric mixer for mashing, but the onion may not mash quite as well. Use onion powder to taste if that worries you.