We made a big pot of this with spicy turkey sausage from the farmer's market for a baby shower recently and it was quickly dispatched; since I had most of the ingredients leftover to make another batch, I think you can see where this is going.
After the sausage is nice and brown and gorgeous (don't rush it!), remove it from the pot and set aside. Don't drain out the oil - instead, dump in your onions and garlic. I add a quarter cup of sherry vinegar to pick up all that nice brown crust that had started to form on the bottom of the pan, and half a teaspoon of red chili pepper for a little more heat.
Cook the onions and garlic down until soft, then add your potatoes and sweet potatoes and continue to saute for a few minutes until everything has started to soften.
Next, add two quarts of broth (veggie or chicken is fine) and let everything come to a boil. Reduce the heat, cover, and simmer until the vegetables are soft and velvety smooth (about twenty to thirty minutes).