Sweet Potato Buttermilk Pie

Name: Sweet Potato Buttermilk Pie
Ingredients: 1 1/2 pounds sweet potatoes (about 2 medium potatoes), peeled and chopped into a 1/2-inch dice; 4 tablespoons unsalted butter, melted; 2 tablespoons fresh lemon juice (optional); 1/2 teaspoon freshly grated nutmeg; 1/2 teaspoon ground cinnamon; 1/2 teaspoon kosher salt; 3 large eggs, separated; 1/2 cup sugar; 2 tablespoons all-purpoe flour; 3/4 cup full- or lowfat buttermilk (or, you can make your own; 1 1/3 x All-Butter, Really Flaky Pie Crust; Whipped cream, for serving