Cream the butter and sugar together in a mixing bowl until pale and creamy. Gradually mix in alternate spoonfuls of beaten egg and flour until all has been added and the mixture is smooth. Stir in the baking powder and lemon rind then spoon the mixture into an 18 x 28 cm (7 x 11 inch) roasting tin lined with nonstick baking paper (see
Make the crumble topping. Put the flour, muesli and caster sugar in a mixing bowl, add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. Sprinkle over the top of the fruit. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 45 minutes until the crumble is golden brown and a skewer inserted into the centre comes out clean.
Leave to cool in the tin then lift out using the lining paper. Cut the cake into 16 bars and peel off the lining paper. Store in an airtight tin for up to 2 days.