Start by getting everything prepared, ready to cook - slice the bacon, peel and dice the onion, carrot, potato and celeriac, and halve and slice the leek (or cut smaller, as needed). The celeriac may need well trimmed to remove all of the outside hairs and fibrous roots. Dice the carrot and celeriac pretty small, with the potato a little larger but still small. Note weights and volumes are after peeling (and dicing for volume).
Warm the butter in a medium-large pot/pan over a medium heat then add the bacon. Cook it for a minute to start to cook and release fat then add the onion. Cook, stirring now and then, for a minute or two so that the onion starts to soften and go opaque then add the remaining vegetables (carrot, potato, celeriac and leek). Stir and cook a couple minutes.
Add the stock, parsley, marjoram and bay leaf and season with a little salt and pepper. Cover the pot and bring to a simmer. Reduce the heat to continue simmering and cook for around 15 - 20 minutes until the vegetables are tender to a knifepoint.
Remove the soup from the heat and blend, either by placing in a blender or using an immersion blender. The soup will be relatively thick, but if it's too thick for your taste, you can add a little extra stock, or else add a little cream to also add a slight creaminess.
Warm the Wiener/Frankfurter sausages in warm water, slice then add a sausage-worth number of slices to each bowl (or less if you are making smaller servings).