Mix the sour cream with the horseradish and season with salt and freshly ground pepper. Dose the horseradish according to taste and heat.
Place the dough on a baking sheet with parchment paper and brush with the cream. Leave an approx. 1 cm wide border free all around.
Peel the beetroot and slice it thinly. Quarter the apple, remove the core and cut the apple crosswise into thin slices. Dice the onion or cut into thin wedges. Spread the vegetables on the tarte flambée. If you like, you can top the tarte flambée with finely plucked bacon slices or ham cubes.
Bake in a preheated oven at 220 ° C for about 15 minutes or until the desired brown color. The baking time can vary depending on the oven.
Instead of cream horseradish, freshly grated horseradish can also be used, in which case be especially careful.