Scrub the sweet potatoes well to get rid of dirt, but do not peel them. Pierce the skin of each sweet potato with a fork to allow steam to escape. You can cut them in half, lengthwise to speed up the process.
Rub the sweet potatoes with oil.
Bake sweet potatoes until done, approximately an hour.
In a medium sautéed pan, combine spam and apple and cook for 2 to 3 minutes. Set aside.
When sweet potatoes are done baking, carefully cut them in half lengthwise then carefully scoop out the flesh while leaving the skins intact. You might want to let them rest at room temperature for 5 to 10 minutes before cutting.
Combine sweet potato flesh, whole milk, mozzarella or emmental cheese, spam, and apple in a stand mixer's mixing bowl fitted with paddle. Whip on medium low speed until the mixture is incorporated and creamy. Add salt and pepper to taste. Approximately 2 to 3 minutes.
The mixture should be light and fluffy, not lumpy.
Scoop the mixture back into the skins and smooth out the tops.
Sprinkle with chopped sage. Serve immediately.
If the sweet potatoes mixture gets thicken overtime, heat it up on a pan and thin with a little bit of milk.