Place the apple slices in a bowl of water with the lemon juice. Set aside until needed.
In a bowl place the cinnamon and 50g each of the butter, sugar and flour. Using your fingertips, rub the ingredients together until the mixture begins to resemble breadcrumbs. Set aside until needed.
Set up your Q for indirect cooking with the trivet and convection tray in place. Preheat on high for 10 minutes.
Beat the butter and sugar using an electric mixer, until pale and creamy. Add eggs, one at a time, and beat well after each addition. Using a large spoon, stir in the flour and mix thoroughly.
Pour the mixture into the casserole dish and place the apples on top. Press down gently to secure them. Scatter the crumb mixture evenly on top of the apples.
Once the Q is preheated, turn to the roast setting. Place the casserole dish on the trivet. Cook for 45-50 minutes or until a skewer comes out clean when inserted into the centre of the cake.
Dust with icing sugar and serve.