Preheat oven to 450 F and place the sweet potatoes on a baking tray. Bake until very tender and soft, about an hour. Remove from the oven, cut in half and scoop out the insides, leaving behind a few millimeters of sweet potato around the edges. Set aside for a side dish or smoothie.
Reduce the oven temperature to 375 F.
In a bowl, combine the apples, lemon juice, cornstarch, cinnamon and vanilla. Place the sweet potato jackets onto a baking sheet and stuff the sweet potato jackets with the apple mixture.
In another bowl, combine the oats, flour, brown sugar, butter and salt. Top the sweet potatoes with the apple mixture then top the apple mixture with the crisp topping.
Bake for 25-30 minutes or until golden brown.
Serve with vegan yogurt or coconut cream, if desired.