Rinse the potatoes under warm water. Place in a large pot, cover with cold water, and bring to a boil. Reduce the heat and simmer until the potatoes are just tender, 25 to 30 minutes. Drain and let cool.
Combine the sugar, 1/2 cup water, nutmeg, lemon zest and salt in a medium saucepan, bring to a simmer over medium heat and cook for 10 minutes. Add 5 tablespoons butter and stir well. When the butter has melted, remove from the heat and beat in the lemon juice.Set aside.
Heat oven to 425 degrees.Peel the sweet potatoes and slice into 1/2-inch disks.Use the remaining tablespoon of butter to grease a shallow baking dish. Lay the sweet potatoes in the dish in a single layer and pour the Leon syrup on top. Bake until bubbling hot, about 30 minutes.