Melt the butter in a large heavy saucepan over moderate heat.
Add the onions, leek, celery, bacon, marjoram, thyme, and nutmeg and stir-fry for about 2 minutes, until nicely glazed.
Reduce the heat to low, cover, and steam for 15 minutes, until the onions are very limp; remove and discard the bacon.
Raise the heat to moderate, add the potatoes and broth, bring to a simmer, uncovered, then adjust the heat so the broth bubbles gently.
Cover and simmer for 40 minutes, until the potatoes are mushy.
Remove the pan from the heat and cool the soup, still covered for 15 minutes.
Puree the soup mixture in two batches by blending in a food processor or an electric blender at high speed for 1 minute.
Return the soup to the pan, mix in the salt and pepper, then taste and adjust the seasonings as needed.
Heat the soup, uncovered to serving temperature, ladle into bowls, and sprinkle each portion with snipped chives.
Servings 0