Heat the olive oil in a large saucepan. Add the onion and garlic, and fry gently for 5 minutes till the onion is soft and golden. Add the celeria and continue to fry for 5 minutes.
Pour in the boiling water and stir in the stock cube.
Cook the soup over a medium heat for 15 minutes or until the celeriac is soft. Add the spinach to the soup and stir well.
Remove the pan from the heat, purée until smooth with a hand blender and season to taste. If you want it to be a little thinner add 200ml milk and warm through again.
Serve in bowls with a swirl of Milbona crème fraîche and some toasted hazelnuts