Preheat your oven to 375°F (190°C). Line a baking dish with a light spray of cooking oil or a touch of butter to prevent sticking.
Wash the sweet potatoes well. Peel if you prefer, then cut them into roughly 1-inch rounds or thick slices.
Arrange the potato pieces in a single layer in the prepared baking dish. It’s okay if they’re a little snug—these will steam and soften as they bake.
In a small saucepan, combine the brown sugar, maple syrup, butter, salt, cinnamon, nutmeg, and water. Warm over medium heat just until the butter melts and the mixture is glossy. You don’t need to boil it; you want a smooth glaze.
Pour the glaze over the potatoes, spooning a bit to coat the tops. You’ll see a nice sheen as the sugars start to melt.
Cover the dish with foil and bake for about 25 minutes. Then uncover and bake another 15–20 minutes, or until the potatoes are tender and the glaze has thickened into a glossy coating.
If you’d like a deeper caramelization, turn the oven to broil for 1–2 minutes at the end, watching closely so nothing burns. Remove from the oven and let rest for a few minutes before serving.