Gently fry the bacon and onions in half the butter; add the broth. Mix in the creme fraiche and bring to a boil. Mix in the mashed potatoes and season with salt, pepper and nutmeg.
Heat the rest of the butter until foamy and toss the sausages in the butter, warming them without frying them.
Add the herbs to the hot soup, bring off the boil and add the sausages just before serving.