Creamy Sweet Potato and Rosemary Soup

My new favorite super-food of 2011 is the sweet potato! This soup is simply amazing and will surely become a favorite in your home this winter.
PS- I will upload a picture soon!

Ingredients
3 tablespoons unsalted butter, at room temperature
3 tablespoons olive oil
3 large or 6 small shallots, thinly sliced
2 to 3 cloves garlic, minced
Kosher salt and freshly ground black pepper
2 pounds (about 2 to 3) sweet potatoes, peeled, trimmed and cut into 1/2-inch pieces
2 (6-inch long) stems fresh (or dried) rosemary
6 cups (or less) low-sodium chicken broth
1/2 cup mascarpone cheese, at room temperature

Directions
(Cut up and prepare sweet potatoes, shallots, and garlic before continuing)
In an 8-quart stockpot, melt the butter and oil together over medium-high heat. Add the shallots and garlic. Season with salt and pepper and cook until soft, about 3 to 4 minutes.

Add the sweet potatoes, rosemary and chicken broth (only to cover the sweet potatoes, if you are using less than 2 lbs. of sp, discard the remaining stock). Season with salt and pepper, to taste. Bring the mixture to a boil, reduce the heat, cover let soup simmer until the sweet potatoes are very tender, about 20 to 25 minutes- no peeking!

Turn off the heat and remove the rosemary stems (if using fresh herbs). Using an immersion blender, blend the mixture until smooth and thick. Whisk in the mascarpone cheese until smooth. Season with salt and pepper, to taste. Keep the soup warm over low heat until ready to serve.