Apple Cranberry Casserole (Bella Monde's)

1 package fresh cranberries
3-4 cooking apples, peeled and chopped
1 cup sugar
3 Tbs flour
 
Topping:
1 stick melted butter
½ cup brown sugar
3 packages instant oatmeal (plain)
1 cup walnuts
 
Bake at 350º for 30 minutes.