This is an extraordinarily good dessert that's very basic and good served warm with vanilla ice cream or whipped cream. If you wish, you can add strawberries to the rhubarb mixture, along with a touch of grated gingerroot.
Cut rhubarb in 1/2-inch pieces. In a large saucepan, combine rhubarb, 1/2 cup brown sugar, cinnamon and 3 teaspoons butter. Cook on medium heat, uncovered, until the rhubarb is tender, about 10 minutes.
Make topping by combining flour, rolled oats, 1/2 cup brown sugar and 1/3 cup butter. Mix with fingers until it resembles small peas. Pour rhubarb into a greased baking dish and sprinkle topping over it. Bake at 375F for 20-25 minutes or until top is browned.