1. Peel, core, and dice the apples into 2cm pieces.
2. Juice the lemon.
3. In a slow cooker, combine the apples, lemon juice, sugar, and cinnamon, tossing to coat evenly.
4. Set aside.

1. In a mixing bowl, rub the butter into the flour using your fingers until it resembles breadcrumbs.
2. Stir in the oats, sugar, and salt.
3. Add 2 tbsp of water and gently shake the bowl to form larger crumble pieces.
4. Spread the crumble over the apple mixture in the slow cooker.
5. Cover with a clean tea towel under the lid to absorb steam.
6. Cook on low for 3 hours until the apples are tender and the crumble is golden.
7. For a crispier topping, transfer the slow cooker bowl to a preheated oven at 200°C (180°C (350°F (175°C)) fan) for 10-15 mins.

Serve the apple crumble warm, optionally with custard, ice cream, or double cream.