Preheat your oven to 375°F
Place the sweet potato rounds in a large pot and cover them with water. Bring to a boil and cook for about 5 minutes until they are slightly tender but still firm
Drain the sweet potatoes and set them aside
In a saucepan over medium heat, melt the butter. Stir in the brown sugar, cinnamon, nutmeg, and salt. Cook and stir until the sugar has dissolved and the mixture is smooth
Remove the saucepan from heat and stir in the vanilla extract
Arrange the parboiled sweet potato rounds in a single layer in a baking dish
Pour the butter and sugar mixture over the sweet potatoes, ensuring they are evenly coated
Add 1/2 cup of water to the baking dish to create some steam and prevent the sweet potatoes from drying out
Sprinkle chopped walnut on top
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes
Remove the foil and baste the sweet potatoes with the syrup from the bottom of the dish. Bake for an additional 15-20 minutes, or until the sweet potatoes are tender and the edges are caramelized
Serve the candied sweet potatoes warm, spooning some of the syrup over the top