Heat oven to 180C/160C fan/gas 4. Peel, core and slice the apples. Grease a large serving dish, about 2.5 litre/3 litre with 25g/1oz of the butter and tip in the apple. Scatter the raisins over the top.
In a separate dish beat the remaining butter and sugar together for 10 mins until the butter turns pale and the sugar starts to dissolve. Beat in the eggs, then carefully fold in the flour with a large spoon. Spread the batter over the apples, then bake for 45 mins until golden. Serve with the custard, below.