Peel the sweet potato and poke a few holes in it. Microwave for about 3 minutes or bake for 20 minutes, so that it is starting to get soft but not cooked.
Slice the sweet potato into thin slices, about 1/4 inch thick.
In a small saucepan, melt the butter over medium heat, then add the maple syrup, spices and salt. Whisk together until smooth.
In a baking dish, lay the sweet potatoes in a single layer.
Brush with some of the butter/syrup mixture.
On top of that, place a layer of apple slices. Brush the apples with more syrup.
Continue with another layer of sweet potatoes, then syrup, then apples and then the last of the syrup. Use a brush to ensure that everything is coated with the syrup.
Bake in the oven for 30-40 minutes, or until the apples and sweet potatoes are tender when pierced with a knife.
Notes
You can use butternut squash in place of the sweet potatoes if you want