Take out all ingredients from the fridge well in advance, they should be at room temperature.
Prepare the crumble for the topping first. Put flour, sugar and a pinch of salt in a wide bowl and mix well. Add the butter one piece at a time and mix it with the flour/sugar mixture using a hand mixer on high speed to make crumbles. The consistency should be thick and crumbly.If the batter is too runny, add a little more flour. Place the crumble in the fridge until ready to use.
Wash and core the apples, then cut into thin slices. Put them in a bowl and mix lemon juice, sugar, scrape out the vanilla pod, the pinch of salt and the cinnamon.
Now grease the baking dish well with butter or vegetable oil. First put the apple mixture into the dish and then, using your fingers, spread the crumble evenly over it in a thick layer. The surface should be well covered.Scatter the rolled oats over the top if you like.
Bake the apple crumble for about 20 minutes until the crumble is a nice golden brown.
Then take it out of the oven, let it cool a little and serve it warm.