Remove the skin from the onion and chop finely. In a medium frying pan, sauté in the warm butter until light yellow.
Add the sour beets and sauté briefly. Add the cider. Cut the apple into quarters and remove the core. Grate into the sour beets using a rösti grater. Add the bay leaf spice and season lightly with salt and pepper.
Put the ribs on the sour beets. Put the lid on and cook on a low fire for about an hour, adding a little cider if necessary.
At the end of the cooking time, remove the ribs and keep warm. Mix the crème fraîche into the sour beets and season them if necessary.
At the same time, lightly roast the mustard seeds in a dry frying pan without adding any fat. Sprinkle over the sour beets. Cut the ribs into slices and arrange on the sour beets.