Mix together the oats, natural yoghurt, rhubarb, ginger, and granulated sweetener.
Place this mixture into the bottom of a 350ml jar.
In a clean bowl, mix together the banana and custard yoghurt, quark, vanilla extract, chia seeds, and granulated sweetener. Beat it well to remove any lumps.
Pour this mixture on top of the oat mixture, then close the lid and refrigerate overnight.