I often make the Bircher muesli, almost to the original recipe. Soak the oats overnight in just enough water to cover. Then in the morning add plain yogurt and a chopped apple. And that's it. I don't find it necessary to add any sweetening. Chopped nuts optional (I don't like them).
The original recipe used cream and lemon juice, or condensed milk (it was invented before refrigeration was widely available, so keeping cream fresh would have been a problem). I do remember making it years ago with condensed milk and it was lovely - might try that again some time when I have syns to spare!