method
Heat the butter in a large sauce pan over a medium-low heat and add the onions. Cook gently for 7-8 minutes or until soft (but not brown). Add the garlic and potato cubes and cook, stirring, for an extra 2 minutes.
Add the stock, increase the heat and bring to the boil. Reduce the heat and simmer gently for about 10 minutes. Add the spinach leaves, increase the heat again and simmer for a further 10 minutes.
Remove from the heat and allow to cool a little. Purée the soup in batches in a blender or with a hand-held blender until smooth. Season with some freshly grated nutmeg.
Add lemon juice and season to taste with salt and freshly ground black pepper. Bring the soup back to the simmer and then stir in the cream.