Preheat oven to 350 degrees. In bowl, mix filling ingredients; turn into a 1-quart capacity pie or baking dish. Rub butter into flour just until mixture resembles coarse breadcrumbs, then stir in sugar. Sprinkle this mixture over rhubarb in the pie or baking dish. Bake for 30 to 45 minutes until topping is cooked and golden. Serve with whipped cream and soft brown sugar, or try stirring a little Amaretto into your cream.