Place the cereal or oatmeal in a microwave-safe bowl. Spread the peeled and cut apple on top. Pour water (approx. 300 - 400 ml) over the whole thing until the apple pieces are about half covered. If you like, you can add a dash of sweetener or some sugar here.
Put the whole thing in the microwave for about 4 minutes on a medium setting (about 600 watts). It is best to put a microwave lid on top. Then let it steep for a few more minutes, then the water should be almost completely absorbed, the apples should be soft and the muesli should be swollen.
Then cut the banana into pieces and mix in. I like it creamy and mix in some yogurt. If you want, you can add nuts, flaxseed (good for digestion) or grains.