Ambrosia Apple Filling:
For the crumb topping: Preheat the oven to 375 degrees F. Butter a 9-by-13-inch pan or 6 to 8 ramekins.
Combine the oats, flour, brown sugar, baking powder, cinnamon and salt in a bowl. Mix the butter into the dry ingredients using a pastry blender or fork.
For the filling: Combine the apples, brown sugar, flour, lemon juice and zest, cinnamon, vanilla extract and salt in a bowl. Pour the mixture into the prepared pan or divide evenly between the ramekins.
Top the apple mixture with the crumb topping and bake until golden brown and bubbly around the edges, 30 to 40 minutes. Allow to cool for 15 minutes, then enjoy.