Preheat oven to 200°C/180°fan/Gas 6. In a bowl, rub butter, oats and almonds to an even crumb. Add sugar and mix through.
Peel, quarter, core and thinly slice all apples. Put in a 2 litre (3½ pint) ovenproof dish. Sprinkle with juice, drizzle with syrup and roughly mix in.