Preheat oven to 375˚F.
Place sweet potatoes onto a large baking sheet and drizzle with 2 ½ tablespoons oil. Season with salt and pepper and toss together.
Roast sweet potatoes for 35 to 40 minutes or until fork tender, tossing halfway through the roasting time.
Remove from the oven and set aside.
In a large saucepan over medium heat add remaining oil and butter.
Add leek, shallots and garlic and sauté for 4 to 5 minutes or until leeks and shallots become slightly translucent. Season with salt and pepper.
Add thyme and sage to the leek mixture and continue to sauté for 3 to 4 minutes.
Transfer roasted potatoes and leek mixture to the well of a blender and top off with 2 ½ cups of stock. Blend until the mixture is smooth and thick enough to coat the back of a spoon. Add more stock as needed.
Pour pureed soup back into the saucepan and season with salt and pepper. Simmer soup for 4 to 5 minutes.
Add a pinch of nutmeg and serve with sour cream, diced pancetta and candied pecans, if using.