Core the apples and cut them into cubes — leave the skins on to save time and reduce waste. Place in a saucepan with the sugar, cinnamon, and water. Cook for 5–7 minutes until just softened.
Rub the butter into the flour, then mix in the oats, sugar, and cinnamon.
Divide the apples between four small ovenproof dishes and spread a generous layer of crumble over them. Bake for 15–20 minutes, until the crumble is golden and the apples are bubbling.
Serve with custard, cream, or a spoonful of yogurt.