Step 1Heat the oil in a large saucepan over medium-high heat. Sauté the onion for 5 minutes, or until soft. Add the garlic and zest, and cook, stirring, for 1 minute more.
Step 2Add the potatoes, stock and 2 cups of water, and bring to the boil. Reduce heat and simmer, covered, for 5 minutes. Add the broccoli and peas. Then simmer, partially covered, for 10 minutes, or until vegetables are tender. Set aside to cool slightly.
Step 3Blend soup in batches until smooth. Return soup to a clean saucepan and reheat until hot. Season with black pepper.
Step 4Combine sour cream and mint. Serve soup with a dollop of the mint sour cream, and sprinkle with the pistachio nuts and extra mint leaves.