Mix together the oats, natural yoghurt, rhubarb, ginger and granulated sweetener.
Place this mixture in the bottom of a Kilner jar (the jar should be able to hold up to 350ml).
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, close the lid and refrigerate overnight.