Peel and chop apples and place into slow cooker. Add lemon and mix through.
In a large bowl, mix together all Filing ingredients and pour over apples.
In a medium bowl, mix together all Topping ingredients, except butter and Grapenut Nuggets/Nuts.
Cut butter into pieces and crumble into the Topping.
Mix in nuts (or Grapenuts, if using) and sprinkle over the filling.
Cover and cook on HIGH for 2 hours (or LOW for 4 hours).
After cooking time is complete, remove slow cooker cover and place sideways if using an oval cooker or resting over cooker, to vent, if using a round slow cooker. Let sit for 30-40 minutes, or until topping has hardened a bit.
Half nuts and Grapenut Nuggets work really well too. Serve with a scoop of ice cream, if desired.