Preheat the oven to 350°F (180°C). Spray a 9x13 baking dish with non-stick spray. Set aside.
Add the peeled and cubed apples to a mixing bowl and add in the sugar, flour, cinnamon, nutmeg, salt, lemon juice and vanilla. Toss together until the apples are well coated.
Transfer the apples to the prepared baking dish and smooth down evenly.
Prepare your topping. Add the flour, rolled oats, sugar and cinnamon to a mixing bowl and mix together. Add in the melted butter and vanilla and mix until well mixed.
Crumble the topping with your fingers over the top of the apples, spreading it in an even layer over the top of the apples.
Place into the oven and bake for 50 minutes until the top is golden brown and the filling is bubbling up the sides.
Remove from the oven and allow to cool on the counter for around 10 minutes before serving with whipped cream or vanilla ice cream.