1. Preheat the oven to 180°C, gas mark 4. Grease and line a 28 x 22cm rectangular cake tin with baking parchment. For the crumble topping, place the flour, cinnamon and cold butter into a large bowl and rub together with your fingertips until the mixture resembles breadcrumbs. Stir in the sugar and nuts then transfer to the fridge.
2. For the cake, sift the flour and baking powder into a bowl and set aside. Using a mixer or electric handheld whisk, cream the butter, sugar and vanilla until pale and fluffy. In a separate bowl, whisk together the milk and eggs. Gradually beat a little flour into the butter mixture, followed by a little of the milk and eggs, alternating until they're all mixed in. Spoon the cake mixture into the prepared tin and level with the back of a spoon.
3. Peel, core and halve the apples, then slice thinly and toss in the lemon juice. Arrange on top of the cake mixture then sprinkle the chilled crumble over the top.
4. Bake for 1 hour or until a skewer inserted into the centre of the cake comes out clean. Transfer to a wire rack to cool then cut into 12 pieces. Great with a spoonful of Waitrose Caramel Sauce, or whipped cream.