Pierce the sweet potatoes several times with a fork and place them on a baking sheet. Bake for 45-60 minutes, or until they are tender and easily pierced with a knife.
While the sweet potatoes are baking, prepare the filling. In a small saucepan over medium heat, combine the chopped cranberries, diced apple, maple syrup, cinnamon, and nutmeg. Cook for about 8-10 minutes until the cranberries burst and the apples soften. Remove from the heat and let cool slightly.
When the sweet potatoes are done, remove them from the oven and let them cool for a few minutes. Once they are cool enough to handle, slice each sweet potato in half lengthwise and scoop out the flesh into a mixing bowl, leaving a small border to keep the skin intact.
Add the cranberries and apple mixture to the sweet potato flesh, along with the Greek yogurt. Mix until well combined, adding salt and pepper to taste. You can adjust sweetness with more maple syrup if desired.
Spoon the filling back into the sweet potato skins, mounding the mixture slightly over the tops.
Return the filled sweet potatoes to the oven and bake for an additional 15-20 minutes, until heated through. If using walnuts, sprinkle them over the tops during the last 5 minutes of baking for a crunchy texture.
Remove from the oven and let cool slightly before serving, garnished with fresh parsley if desired.
Notes
You can use a dairy-free yogurt alternative if desired.