Heat the butter in a saucepan, add the onion and fry gently for 5 minutes until softened. Add the potato, cover and cook for 10 minutes, stirring occasionally.
Pour in the stock, add the nutmeg and salt and pepper then bring to the boil. Cover and simmer for 20 minutes until the potato is soft. Reserve a few tiny spinach leaves and add the rest to the pan. Re-cover the pan and cook for 5 minutes until just wilted.
Purée the soup in batches in a blender or food processor until smooth, then pour back into the saucepan, mix in the milk and set aside.
Cut the haddock into two pieces, cook in a steamer for 8–10 minutes until the fish flakes when pressed with a knife. Put the quail's eggs into a small saucepan of cold water, bring to the boil and simmer for 2–3 minutes, drain, rinse with cold water and peel off shells.
Mix the 2 hen egg yolks with the double cream. Stir into the soup and bring just to the boil, still stirring. Taste and adjust the seasoning if needed. Flake the fish, discarding the skin and bones, make small mounds in the base of 6 shallow serving bowls and top with the quail's egg halves. Ladle the soup around the fish and eggs and garnish with tiny spinach leaves and pepper.