How to Make It
Preheat oven to 375°F. Toss together finely chopped rhubarb and flour in a medium bowl to coat. Set aside.
Beat together brown sugar and softened butter with a heavy-duty stand mixer on medium speed, using the paddle attachment, until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating well after each addition. Add sour cream and vanilla, and beat until combined.
Whisk together flour, baking soda, salt, cinnamon, and nutmeg in a medium bowl. Gradually add flour mixture to butter mixture, beating at low speed just until combined, about 20 seconds. Fold in chopped rhubarb mixture. Spread batter evenly in a greased (with shortening) and floured 9-inch round springform pan. If desired, top with rhubarb pieces in a decorative pattern, and brush pieces with melted butter. Sprinkle top evenly with sugar.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 1 hour to 1 hour and 10 minutes. Let cool in pan 15 minutes on a wire rack; remove sides of pan, and let cool completely, about 1 hour.