Mix cornstarch with the water. Combine water mixture with the rest of the filling ingredients (except the vanilla) in a saucepan. Cook over medium heat until liquid looks clear. Remove from heat and add vanilla. Cool.
Combine Oatmeal, flour, brown sugar, cinnamon and soda. Cut in the softened butter as you would a pie dough (crumbly texture). Then add nuts. Reserve 1 cup of mixture and press remaining amount in a 13 x 9 baking pan. Spread cooled filling mixture on crust and then top with the remaining reserved crust mixture. Pat top just slightly. Bake at 350 for about 30 minutes or until golden brown. Cool completely before cutting. Enjoy!