1. Heat the butter in a large pan and saute the peas, spring onions and spinach until the vegetables are tender.
2. Add the water/vegetable stock, salt, pepper and nutmeg and bring to a boil, cool.
3. Put the soup in an electric blender and blend until smooth.
4. Return the soup to the pan, heat thoroughly and pour into bowls or cups.
5. Sprinkle extra nutmeg on top, garnish with a swirl of yogurt and serve hot.