Heat your oven to 400 degrees. Then start by mixing the flour, brown sugar, oats, and cinnamon together in a medium bowl. Add the butter and use your hands or two forks to mix everything together until small clumps form. Then add the walnuts.

Combine the apples, sugar, flour and vanilla extract in a bowl and toss together. Either pour into one baking dish or divide into 6 servings in individual baking dishes.

Sprinkle the topping evenly over the apples. Bake for 40 minutes and let cool 10 minutes. Serve warm with ice cream.