Heat the oil in a large saucepan over medium heat. Add the onion and cook, stirring, for 5 minutes or until soft. Add the garlic and cumin and cook, stirring, for 30 seconds or until aromatic.
Increase heat to high. Add the stock and cream and bring to the boil. Add potato and reduce heat to medium. Simmer, uncovered, for 10 minutes or until potato is tender. Add spinach and cook for 2-3 minutes or until the spinach wilts. Set aside for 10 minutes to cool slightly.
Place half the spinach mixture in the bowl of a food processor and process until smooth. Transfer to a clean saucepan. Repeat with remaining spinach mixture. Place over low heat and cook, stirring, for 4-5 minutes or until heated through. Remove from heat. Taste and season with salt and pepper.
Ladle soup among serving bowls. Top with a dollop of sour cream and season with pepper and croutons (Mark made his own by stir frying cubes of a french stick with garlic and butter). Serve with crusty bread and lashings of butter.