Filling:
In a medium saucepan combine sugar and cornstarch. Stir in rhubarb.
Cook and stir over medium heat, until mixture comes to a boil and thickens. Remove from heat and cool.
Cake:
Combine milk and lemon juice. Set aside.
Combine flour and sugar. Cut in butter until mixture is crumbly ( a
food processor is great for this). Remove 1/2 cup of this mixture and set it aside to use for topping.
To the remaining flour mixture add
baking powder, baking soda, and walnuts.
Combine egg with milk mixture. Add to the dry ingredients and stir or process until just moistened.
Spread 2/3 of the batter over the bottom and part way up the sides of a greased 9 inch