Heat 2 tablespoons of olive oil in a pan and toast the pieces of seitan. Take the seitan out of the pan, season with salt and pepper.
Now sauté the onion rings in the olive oil until they are slightly translucent (do not let them turn brown), add the vegetable stock and mushrooms. Simmer over medium heat for a few minutes, then add the cucumber and beetroot strips and the mustard. Mix in the crème fraîche, bring everything to the boil. Let simmer for a few minutes. Thicken if desired with the flour mixture, season with salt and pepper.
Ribbon noodles go wonderfully with it as a side dish.