Peel and chop the onion and beetroot. (Attention! They stain strongly, please wear disposable gloves.) Wash the apple. Cut and dice slices around the core. Mix 1 tablespoon with lemon juice and put aside.
Heat the oil in a pot. Fry onion, beetroot and apple for about 5 minutes. Add 3/8 l water and broth, bring to the boil. Cover and cook for about 20 minutes until soft.
In the meantime, wash the chives, shake them dry and cut them into small rolls. Mix sour cream, horseradish and chives. Season to taste with salt and pepper. Cut bread into thin slices and roast in a toaster.
Finely puree the soup with a hand blender. Season to taste with salt and pepper. Serve. Stir the horseradish cream into streaks and sprinkle with apple cubes. Add the flatbread.