Grease or line an 8"/20cm round cake tin and preheat the oven to 375F/190C.
To make the topping, stir together both sugars, cinnamon and flour. Pour in the melted butter and fluff it up with a fork until you've got a crumb-like mixture. Leave to one side.
For the apple filling, toss together the apple slices, sugar, cinnamon and lemon juice. Leave to one side.
For the cake, stir together the flour, sugar, baking powder, cinnamon and baking soda.
In a separate bowl, whisk together the milk, vanilla, oil and egg until smooth.
Pour the wet ingredients into the dry and gently combine with a spoon until the mixture just comes together - don't over-mix.
Pour the cake batter into the lined/greased pan.
Layer the apple slices on top of the cake batter.
Sprinkle the crumble layer on top of the apple slices.
Bake for around 45 minutes, until the crumble topping is golden brown. Leave to cool completely in the tin before slicing and serving or covering and storing in the fridge for up to five days.