Peel, core and slice the apples, then scatter over the bottom of a shallow ovenproof dish and sprinkle with the caster sugar. Drizzle with the lemon juice.
Melt the butter in a saucepan on the hob and stir in the cinnamon, then the oats and muscovado sugar until evenly mixed. Spoon the mixture over the apples. Cover the dish with foil and place on the lower rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for 40 minutes, removing the foil for the final 10 minutes of cooking time to colour the crumble topping. Serve with vanilla ice cream.