It's a bit difficult to be sure what his original recipe was but certainly from the sources I've found, oats and apples seem to be obligatory. Opinions seem to differ on whether it should include milk or yogurt or even cream. Many include lemon juice which I don't due to the amount of other fruits I include which even at their ripest, also have a slight acidity.
Not everyone sweetens the recipe either. I included a touch of honey and I will leave that up to you whether to include or not. It's not a necessity if the fruit is ripe, in fact you could easily make this vegan by using nut milk and missing out the honey.
This is my massively fruit filled take on Bircher Muesli. I hope Dr Bircher-Benner would approve.
Bircher Muesli - Serves 1
1 small (e.g. "fun size") apple or half a large apple
2-3 tbsp oats, use gluten free oats if liked
Milk, use dairy milk if liked or nut milk if you prefer
1 handful blueberries
1 handful strawberries
1 handful raspberries
1 tsp honey (optional)
Coarsely grate the apple including the skin back to the core. Add the oats then enough milk to your liking. The oats will absorb a little as you prep the other fruit.
Prepare the fruits and place into the bowl. Top with a drizzle of honey if liked and serve. If allowed to stand for longer, the oats will soften further.