Preheat oven to 250C. Cut duck through the bone into 16 pieces, place in a single layer in a large roasting pan, roast until golden (12-15 minutes). Remove from pan, pat off excess fat with absorbent paper and combine in a large saucepan with onion, carrot, celery, garlic, bay leaves and 3 litres water. Bring to the boil over medium-high heat, reduce heat to low-medium and simmer, skimming occasionally, until meat falls from the bone (1½-2 hours). Remove duck, coarsely shred meat (discard skin and bones) and return to broth. Add peas, thyme and cinnamon and simmer until peas are tender (2-3 minutes).
Meanwhile, for Spätzle, bring a large saucepan of salted water to the boil over high heat. Mix flour, eggs, butter and 60ml water in a bowl until smooth and combined, season to taste with sea salt, black pepper and nutmeg. Transfer to a piping bag fitted with a 5mm-nozzle and pipe in batches into boiling water, cutting at 2cm-intervals with kitchen scissors. Cook until Spätzle rise to the surface (1-2 minutes), remove with a slotted spoon and add to broth, ladle among small bowls and serve immediately.