Cold weather comfort food! Make sure the liquid doesn't evaporate too quickly; otherwise, add a little water at the end of cooking to ensure a thick, velvety sauce to coat the vegetables.
Clean leeks. Slice the whites into 3-inch sections. Chop greens into large pieces.
Wash sweet potatoes thoroughly. Cut into pieces equal in size to leeks.
Put a thick-bottomed pot on medium heat. Add duck fat and mustard seeds. When they begin to sizzle, add leek greens and stir well.
Place sweet potatoes and leeks in the pot. Add thyme, sage, fennel, bay leaves, salt, pepper and nutmeg. Pour in chicken stock and cover. Reduce heat to low after about 5 minutes. Let steam in the pot until vegetables are tender, 15-20 minutes.
Add honey and vinegar. Shake the pot to coat the vegetables. Cook a few minutes more, uncovered, to reduce the juice. Let stand a few more minutes before serving.