Remove the seitan from the marinade and briefly sear it in olive oil until golden on both sides. Set aside.
In the same pan, sauté the chopped onions, carrots, and parsley root (or celery) until soft.
Add tomato paste, prošek or date syrup, dried prunes, chopped pickle, mustard, and spices. Let everything cook for a few minutes to release the aromas.
Simmering
Return the seitan to the pan. Add vegetable broth and some of the marinade.
Simmer on low heat for 45–60 minutes, stirring occasionally. (If using soy medallions, stir gently so they don't fall apart.)
You can partially blend the sauce with an immersion blender at the end to thicken it, leaving some texture.
For an extra depth of flavor add near the end of cooking a slash of balsamic vinegar, a pinch of cinnamon and nutmeg. This will enrich the flavor beautifully.