Preheat oven to 375°F. Peel sweet potatoes and slice into 1/2-inch thick rounds. Grease a 9x13-inch baking dish with nonstick spray.
Arrange sliced sweet potatoes in the prepared baking dish. Sprinkle with salt and toss to coat evenly.
In a medium saucepan over medium heat, combine butter, water, maple syrup, brown sugar, cinnamon, nutmeg, and ginger. Stir until butter melts and sugar dissolves.
Once melted, stop stirring and let the mixture come to a gentle boil. Boil for 2 minutes without stirring.
Remove from heat and stir in vanilla extract and orange zest (if using).
Pour the sauce over the sweet potatoes and toss to coat each slice evenly.
Bake uncovered for 20 minutes. Remove from oven, stir well, then cover with aluminum foil.
Bake for another 20 minutes. Remove, stir again, and bake for a final 20 minutes (total 60 minutes).
Remove from oven and let rest uncovered for 10 minutes. The sauce will thicken as it cools.
Garnish with chopped rosemary and flaky sea salt if desired. Serve warm.