Poke the sweet potatoes a few times all around and place on a baking sheet. Bake at 375°F for 30-50 minutes, or until completely tender. Set aside to cool slightly.
Remove casings from sausage. Place sausage meat in a fry pan and sauté until fully cooked. Set aside.
Add butter to skillet and over medium heat sauté the leeks with the sage, thyme and about ½ tsp salt. Stir frequently until tender and golden. Set aside.
Cut the top ¼ off each sweet potato, then carefully scoop out the flesh and place into a large bowl. Mash the scooped out flesh until smooth, then gently stir in the sour cream, sausage and leeks.
Spoon the filling into the potato shells, top with cheese, and place in a baking dish. Broil for about 5 minutes or until cheese is melted.
Notes
For a lower calorie version, slice the sweet potatoes in half lengthwise once they are baked then proceed as directed (giving you 8 portions instead of 4).