Preheat oven to 170C and prepare a 23cm round springform tin by buttering it and dusting it with flour.
Beat butter, caster sugar and lemon rind in an electric or stand mixer until pale and fluffy (2-3 minutes), scrape down sides of bowl as you go to ensure it is all equally combined, then beat in egg. Sift in flours, fold to combine, stir in milk and mascarpone, spoon into the springform tin and gently smooth the top.
For mascarpone filling, whisk ingredients in a bowl until smooth, then spoon over cake batter and smooth top.
For apple topping, toss apple slices in lemon juice and arrange, overlapping slightly, on top of mascarpone mix. Brush with melted butter, scatter with brown sugar, then bake until golden and risen – about 1 hour. Cool in tin for 20 minutes, then run a sharp knife around sides of tin to loosen cake, remove sides of tin, cool cake to room temperature and dust with icing sugar. The apple and mascarpone cake will keep stored in an airtight container for 2 days.