Finely chop the onion, garlic and celery.
Slice or shred the fennel and chop the stalks and fronds.
Heat the oil in a pan, then gently sauté the chopped vegetables for 5 minutes. Add the stock, bring to the boil. Turn the heat down and simmer, under a lid, for about 15 minutes.
Meanwhile, rinse and drain the spinach.
When the vegetables are soft, add the spinach to the soup, turn off the heat and rest, under cover, for 2-3 minutes until the spinach is wilted. Blend until you get a smooth, creamy soup.
Serve with chopped chives and a dollop of yoghurt if liked.