Preheat the oven to 180°C/ 160°C fan/ gas mark 4/ 350°F
Line the base of the baking tin with baking parchment and/or lightly grease the tin with butter
Peel and core the apples, and slice thinly
Mix 50g caster sugar with the sultanas
Zest the lemon
For the Apple Sponge Cake:
Beat all the cake ingredients together (easiest with a mixer or processor)(Butter, sugar, egg, lemon zest, flour, baking powder). Note that this is a stiff mixture, more like a dough than a batter.
Spoon half (or slightly more) the sponge mixture into the cake tin and level with a palette knife or the back of a spoon. You will have a thin layer of cake mixture, enough to cover the base of the cake tin
Evenly cover the mixture with the sliced apples. Sprinkle the sultana/sugar mix over the apples. Press down with your fingers or the back of a spoon to settle the fruit
Cover with the rest of the sponge mixture. Level the top and sprinkle the remaining 30g caster sugar over. Don't worry if some apple shows through
Cook for 50-60 minutes, until the cake surface springs back when you touch it lightly with your fingertip or a cocktail stick comes out clean
Cool in the tin or serve warm with custard or cream for pudding.
Notes
Store for up to a week in an airtight container in the fridge.