Combine all ingredients for the bircher muesli in a large bowl and mix thoroughly. Cover in cling film and place in the fridge overnight. Store covered in the fridge for up to 3 days. To make the rhubarb compote, place all the ingredients into a saucepan and bring to a simmer. Simmer for 10-15 minutes until the rhubarb is soft and broken down. Layer together with the oats and enjoy!