In medium sized bowl, mix flour, oatmeal, brown sugar, butter and cinnamon until crumbly. Use a pastry cutter to make this very easy.
Press 1/2 of the mixture into a greased
8x10x2 inch baking pan.
Top with the rhubarb.
Combine the sugar, cornstarch, water and vanilla in small saucepan. Heat slowly and simmer until thickened. Pour over the rhubarb. Cover with remaining crumb mixture. Bake at 350 degrees for 1 hour. (Good idea to put a cooking sheet on the rack below the baking pan to catch any drips.)
Let cool until just warm. Wonderful served with a scoop of vanilla ice cream.
Strawberries, blueberries, raspberries are wonderfull additions -just don't tell my mother!