Peel, core and chopped in 1cm pieces. Place the pieces in a large bowl along with cinnamon, brown sugar and lemon juice. Stir well to coat the apples. Spread the apples evenly on the base of a large rectangular casserole dish that fits inside the microwave. Set aside while you make the crumble topping.
Place butter in a medium bowl, microwave on Medium power for 1 minute. Add in brown sugar, rolled oats, flour, cinnamon and salt and stir well to combine. Spread the crumble mixture evenly over the apples.
Cook on High for 15 minutes. Allow to rest for 5-10 minutes before serving with ice cream.