This is a little tricky, but well worth the effort. Once you get the hang of slicing the potatoes, the rest is easy. It can be made ahead. It looks great, is delicious, and it's quite healthy. From Martha Stewart.
Preheat oven to 375. Wrap 1 sweet potato in parchment, and then in foil. Pierce several times with a fork. Bake until tender, about 1 hour. Let cool. Reduce oven temperature to 350.
Meanwhile, peel remaining sweet potatoes. Using a mandoline or a chef's knife, cut potatoes lengthwise into very thin slices until you have 30 slices. Trim each to a 2-by-4-inch rectangle.
Bring a large pot of water to a boil. Add half the sweet potato slices, and cook until tender, about 2 minutes. Using a slotted spoon, place slices on a baking sheet to cool slightly. Repeat.
Remove peel from baked sweet potato, and puree flesh in a food processor until smooth. Add cottage cheese, and puree until smooth.
Transfer potato-cheese mixture to a large bowl. Stir in apple, chives, grated Parmesan, salt, and pepper.
Coat a 9-by-13-inch baking dish with oil. Place 1 heaping tablespoon filling in center of a sweet potato slice, and roll up. Place, seam side down, in dish. Repeat. (Cannelloni can be refrigerated, covered, overnight; bring to room temperature before baking.)
Brush oil over cannelloni. Bake until heated through, 10 to 15 minutes. Sprinkle with walnuts and shaved Parmesan.