In a large skillet over medium heat, add the diced sweet potatoes. Cook them for about 5-7 minutes until they start to soften, stirring occasionally.
Stir in the sliced apples into the skillet with the sweet potatoes. Cook for another 3-5 minutes until the apples begin to soften but are still firm.
In a medium bowl, combine the rolled oats, brown sugar, flour, cinnamon, nutmeg, salt, and nuts (if using). Pour in the melted butter and vanilla extract. Stir until the mixture is well combined and crumbly.
Once the sweet potatoes and apples are softened, sprinkle the oat mixture evenly over the fruit in the skillet.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the topping is golden brown and the fruit is bubbly.
Remove the skillet from the oven and let it cool slightly. Serve warm with a scoop of vanilla ice cream or a dollop of yogurt, if desired.