Combine the following in a bowl: flour, oats, brown sugar, and baking soda
Add 1 cup + 2 Tablespoons butter - cut into mixture with pastry cutter or knife or fingers until youi have coarse crumbs
Keep 2 cups for topping
Put the rest of the crumbs in the bottom of a 9 x 12" greased pan
Peel and slice apples and put them in layers on top of the crumbs
Set aside.
In saucepan, combine the following: sugar, cornstarch, water, vanilla, and 2 Tbsp. butter
Bring to a boil and stir for about 2 minutes or until it is thick and bubbly
Spread sauce over the apples
Top with the remaining crumbs
Bake at 350 for 35-45 minutes or until lightly browned