Preheat the oven to 190C/gas 5/fan 170C.
Peel, core and slice the apples and put them in a 1.5 litre pie dish. Pour over the lemon juice and brown sugar, and stir together.
To make the topping, tip the flour into a large bowl. Add the butter so that it is evenly distributed through the flour. Stir in the muesli and scatter evenly over the apples to cover them.
Bake for 40-45 minutes until golden and crunchy on top. Leave to cool slightly for 5-10 minutes before serving.
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