Recipe courtesy Sara Moulton

2 tablespoons butter
1 cup chopped onions
8 medium sweet potatoes
3/4 cup milk
4 tablespoons butter
1/4 cup candied ginger, chopped, or to taste

Saute onions in butter until translucent. Set aside. Peel potatoes and cut into 1-inch pieces. Place potatoes in saucepan with cold salted water to cover and boil, covered, until very tender, about 15 minutes. Drain in colander. In a food processor, puree potatoes, milk, butter and Sauteed onions until smooth. Season with salt and pepper. Sprinkle with chopped candied ginger.

(c) Food Network, Sara Moulton

Here's another winner I busted out at Thanks Giving. I modified it slightly in that I used a potato ricer so as not to over mash and render them gluey (originally found on http://www.allrecipes.com/ and tips specifically from here)

Ingredients

2 pounds potatoes 
4 cloves garlic 
1/4 cup freshly grated 
   Parmesan cheese 
1 tablespoon butter or margarine 
1 tablespoon chopped fresh rosemary 
1/2 cup Evaporated Lowfat Milk 
salt and ground black pepper to taste

Directions

1 PLACE potatoes and garlic in large saucepan. Cover 
   with water; bring to a boil. Cook over medium-high heat 
   for 15 to 20 minutes or until potatoes are tender; drain. 
2 RETURN potatoes and garlic to saucepan. Mash potatoes and
 garlic together with a potato ricer. Add cheese, butter and 
   rosemary; stir until smooth (but not too much). Gradually stir in evaporated milk. Season with salt and pepper.