1) Peel and cube yams and boil them in salted water until they are very tender, about 12-15 minutes. Drain yams and mash in a bowl.
2) You can cook shells in same water. Cook until al dente and then remove and drain shells.
3) Dice onions, sage, and garlic and saute in butter over medium heat until onions are soft, about 5-6 minutes. Don’t brown the onions. Add red pepper flakes and a pinch of salt.
4) Add onion mixture to mashed yams with greek yogurt, cream, and walnuts. Stir together and season with salt and pepper.
5) Add half of the tomato sauce to the bottom of a 9×13 baking dish. Working with one shell at a time, fill shell with a few spoonfuls of filling. You can stuff them pretty full. It’s also okay if they rip a bit.
6) Arrange stuffed shells in baking dish. Once the dish is full, top each shell with extra sauce and a slice of mozzarella.
7) Cover dish loosely with foil and bake for 30 minutes at 350 degrees. Then uncover and bake dish for another 15 minutes at 400 degrees.
8) Serve shells garnished with fresh chopped chives.