Mix the milk, yogurt and oats in a bowl until well combined.
Smash the banana and add to the mix.
Coarsely grate the apple with the skin on and mix in as well.
At this point you can add some honey or Maple syrup for more sweetness and add chopped dried fruit and nuts/seeds.
Either cover the whole bowl with some cling wrap or fill mix into single serving jars and cover them. Store in the fridge overnight.
The next morning top the Bircher Müsli with cut fruit (e.g. figs, kiwi, mango...), drizzle some honey or Maple syrup over it, dust with some cinnamon (or cardamom). When I feel fancy I also thinly grate some dark chocolate over it.
A single serving jar can also be taken to work for lunch, especially if you like it warmed to room temperature.