Remove the outer cabbage leaves and the stalk. Cut cabbage into 2cm squares.
Dice the onion. Fry bacon in a large pot, add the onion and sautée them.
Add the cabbage and sautée for a few minutes while stirring. Pour in 1 liter of water and bring to a boil. Stir in stock cube, mustard seeds, bay leaves and thyme. Cover and simmer for 10 minutes.
Peel potatoes, cut into cubes and add to cabbage, cover and simmer 20 minutes. Add sausage and simmer 15 minutes more.
Remove bay leaves and sausage and stir cream into the soup. Season with salt and pepper.