Preheat and prep: Heat the oven to 400°F (205°C). Scrub the sweet potatoes, dry them, and pierce each a few times with a fork.
Place on a parchment-lined baking sheet.
Bake the sweet potatoes: Bake for 45–60 minutes, until very tender when pierced. The skin should wrinkle slightly and juices may caramelize on the pan.
Make the apple filling: While the potatoes bake, peel (optional) and dice the apples into small cubes. In a skillet over medium heat, melt 2 tablespoons butter.
Add apples, maple syrup, cinnamon, nutmeg, vanilla, and a pinch of salt. Cook 6–8 minutes until the apples are tender but not mushy. Set aside.
Mix the crumble: In a bowl, combine oats, nuts, flour, brown sugar, cinnamon, and salt.
Add the cold butter and rub it in with your fingers or a fork until the mixture forms coarse crumbs.
Split the potatoes: Let baked sweet potatoes cool 5 minutes. Cut a slit down the center of each and gently push the ends toward the middle to open them up. Fluff the inside with a fork.
Stuff with apples: Spoon warm apple filling into each potato, pressing it slightly into the flesh.
Add the crumble: Top each potato generously with the oat mixture, pressing lightly so it sticks.
Return to oven: Reduce oven to 375°F (190°C).
Bake stuffed potatoes for 10–12 minutes, until the topping is golden and crisp.
Serve: Let cool a couple minutes. Finish with a drizzle of maple syrup or a dollop of yogurt, whipped cream, or ice cream if you like.