Preheat the oven to 180°C.
Prepare the crust. In a bowl, add the chopped butter and all other crust ingredients. Quickly mix the ingredients with your hands until the dough is formed. Roll the dough and transfer it to your baking tin. Place the prepared crust into the oven and pre-bake for 10-12 minutes, set aside.
Prepare the custard according to the instructions on the package, using 450 ml of milk (instead of 500 ml) and one tablespoon of sugar. Set aside to cool.
Prepare the crumble. In a bowl, add finely chopped butter and all other crumble ingredients. Mix all the ingredients with your hands until a crumble is formed.
Wash the rhubarb stalks and cut them into smaller pieces.
To assemble the tart, transfer the prepared custard on the pre-baked, cooled crust, then cover the custard layer with the sliced rhubarb. Sprinkle the crumble mixture on top. Bake your tart for about 30 minutes or until the top of the crumble is crisped. Enjoy your meal!