Preheat oven to 350 degrees
Butter an 8x8 baking dish, or spray with non-stick cooking spray, and set aside
In a mixing bowl, add chopped.sliced peeled apples, berries if using, brown sugar, cinnamon and lemon juice. Sprinkle flour, stir to combine, then transfer to prepared baking dish.
In a separate mixing bowl, add topping ingredients (brown sugar, oat, flour, cinnamon, salt and diced cold butter). Use a pastry cutter, or two forks or even your hands to mix the butter into the oat mixture, until the butter is well integrated into the oat mixture. You can add a bit more oats if you do not think you will have enough to cover the apples. You do not need a thick layer, but it should cover the apples well.
Sprinkle the oat mixture over the apples in baking dish, and gently pat to even it out, I like to drizzle the maple syrup on top.
Bake 40-50 minutes, until golden brown and bubbly.
Serve warm with ice cream or by itself. Enjoy!