Place a dutch oven on a low heat and gently cook onions with some oil and salt until soft.
Pour in chicken stock and bring to a boil. Add potatoes and cook until soft.
Remove from the heat and add the spinach.
In a bowl whisk the eggs and then add the lemon juice and stir. Then add garlic, parmesan, mustard, worcestershire sauce and yoghurt. Mix until combined.
Temper the mixture with a couple of spoonfuls of the soup to bring the egg mixture to a warm temperature and pour into the soup whilst stirring. Make sure your soup isn't really hot otherwise it will curdle.
Place the soup back onto a LOW heat to warm through. Add the chicken.
Once warm and almost at a simmer, taste to see if there is enough lemon and seasoning and when happy with it, remove from the heat and serve with rosemary croutons, parmesan, a drizzle of oil and lots of black pepper.
