Defrost the spinach. Clean the leek and cut into rings.
Bring the broth to the boil. Add half a teaspoon each of salt and pepper as well as the leek and cook over a low heat for 20 minutes. Add the spinach and cook for another ten minutes. Remove the soup from the hotplate and puree. Crush the garlic and coriander in a mortar. Steam the garlic pulp for two minutes in hot butter over low heat. Add to the soup and cook for five minutes. Season to taste with salt and pepper.