Muesli is eaten raw with no added sugars or oil. Eat it soaked in milk for 15 minutes or made overnight like we did here. Overnight oats or Bircher Muesli is simply oats mixed with milk and/or yogurt, stored in the refrigerator overnight and topped with fruit. Overnight oats are prepped earlier on Sunday and stay good for up to four days. Overnight muesli is typically good for the just the one night.