In a bowl mix the flour, butter and sugar with your hands until crumbs form. Put it in the fridge to chill for about 20 minutes (or 5 minutes in the freezer).
Apple Filling
Peel the apples and then cut them into 8 or 10 pieces.
In a saucepan, add the apple slices, pitted plums, butter, vanilla pod and sugar. Sauté over medium heat until the apples begin to brown.
Pour 3 tablespoons of water and lower the heat covering the saucepan with a lid. Let it cook and stir occasionally until the apples are tender.
Assembly
Pour the apple filling into a baking dish and sprinkle the crumbs on top.
Bake for 45 minutes. You will see that it is ready when the apple layer is bubbling and the crumble topping is golden brown. Et voilà… You just made a delicious apple crumble pie!