Preheat the oven to 200°C. Line a muffin tin with paper cases.
Prepare the crumble by rubbing the butter, flour and sugar together with your fingers until you get a mixture resembling breadcrumbs. Set aside.
Sieve the flour, baking powder and salt into a large mixing bowl and mix with the sugar.
In another bowl mix together the butter, milk, egg & lemon zest.
Peel and core the apples, then roughly chop them into bite-sized pieces.
Add the wet mixture to the flour mixture, then gently fold in the fruits.
Spoon the batter into the muffin cases (about two-thirds up) then cover with the crumble topping.
Bake for 20-25 minutes until golden brown, then cool the muffins on a wire rack.