This recipe is excellent and i tried it a couple of times myself, too.

Shifted flour 1 cup
Brown sugar 1 cup
Melted butter 1/2 cup
Cinnamon 2 tsp
Oatmeal 3/4 cup
Diced Rhubarb 4 cups
Sugar 1 cup
cornstarch 2 tsp
Water 1 cup
Vanilla extra 1 tsp

Mix flour, brown sugar, butter, cinnamon, salt and oatmeal until
crumbly. Press 1/2 of crumbs into a buttered 9" square pan, cover with
diced apples (or rhubarb). Combine sugar, cornstarch, water and
vanilla. Cook until thick and pour over fruit. Top with remaining
crumbs. Bake in moderate oven, 350 degrees 1 hour. Cut in squares and
top with whipped cream if desired.