Lay sliced apples into the bottom of the slow cooker. Squeeze juice of half a lemon over apples and stir to coat.
To a small bowl, stir together the sugar, flour, and cinnamon and pour over the apples. Stir to evenly coat.
To another small bowl, stir together the flour, oats, brown sugar, salt, and cinnamon. Once evenly mixed, add in the cold, cubed butter. Use a pastry cutter, or a fork, to incorporate the butter to the oat mixture. It will be crumbly.
Once combined, sprinkle the topping evenly over the apples.
Line the top of the slow cooker basin with paper towel and place the lid on top (this helps collect condensation and keep the topping crisp). Cook on HIGH for 1.5 hours. Remove the lid and cook for another 30-60 minutes to allow the topping to crisp.
Serve warm or cold with whipped cream or vanilla ice cream.
Granny Smith apples work well - this was 3 large apples for me - Fuji, Braeburn, Jonagold will also work.Store leftovers in an airtight container in the fridge for up to 3 days.