Preheat oven to 375.
Butter sides and bottom of cassarole dish.
Place sliced apples or filling in dish and spread out evenly.
In medium bowl, mix flour, oats, brown sugar, cinnamon and butter until crumbly.
I like using a fork or hands to mix it into pea size crumbs.
Sprinkle evenly over apples.
Bake for 20 minutes or until apples are tender and top is golden brown.
Serve warm or cold :)