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• 800g floury potatoes (such as coliban), coarsely chopped
• 400g parsnips, coarsely chopped
• 1 tablespoon vegetable or olive oil
• 2 cloves garlic, crushed
• ⅓ cup extra-light sour cream
• 2 tablespoons chopped flat-leaf parsley
• 2 teaspoons finely grated lemon zest, plus extra strips, to serve