Preheat the oven to 180C/160C Fan/Gas 4. Grease the sides of a 20cm/8in square cake tin and line the base with baking paper.
To make the crumble topping, rub the flour and butter together with the tips of your fingers until the mixture resembles rough breadcrumbs. (You can also pulse the ingredients together briefly in a food processor.) Stir in the sugar. Put in the fridge until needed.
To make the cake, beat the sugar and butter together until light. Beat in the eggs one at a time, adding half the flour after you’ve beaten in the first egg, then stir in the rest of the flour and the baking powder.
Peel the apples, cut into quarters, cut out the core and cut each quarter into four long slices. Mix with the lemon zest and juice and cinnamon.
Spoon the cake mixture into the prepared tin. Place the apples on top and cover with the crumble topping. Bake the cake for 40 minutes, or until a skewer inserted into the centre comes out clean. Leave to cool in the tin for 10 minutes, turn out the cake carefully onto a wire rack and then turn it over onto another wire rack so it’s crumble-side up.
Serve the apple crumble cake at room temperature or warm with cream.