For the onion mustard soup, peel the onions and cut them into fine rings.
Heat a mixture of oil and butter in a pot and roast the onion rings in it until golden. Deglaze with must and add soup. Put herbs and garlic clove in a spice bag or tea bag and add to soup, season with salt and pepper. Bring to a boil and then simmer at medium temperature for 15-20 minutes.
Remove the spice bag and serve the onion-must soup hot.