Preheat oven to 375 F. Combine rhubarb, 6 tablespoons flour and 2 cups granulated sugar and place in bottom of a greased 9x13-inch baking dish. In medium bowl, combine brown sugar, remaining flour and oats. Cut in butter until well combined. Sprinkle over rhubarb mixture. Bake 40 minutes or until golden. Serve warm with vanilla ice cream or a bit of heavy cream. Optional: use apples in place of rhubarb and add 1 teaspoon nutmeg and 1/2 teaspoon cinnamon to topping.