Place the lemon zest and the olive oil in a small bowl in the microwave for one minute.
Stir well,then set aside to infuse.
Heat the butter in a saucepan.
Add the leeks,onions and celery and garlic.
Cook over a low heat for 10 minutes until soft.
Add the potatoes and stir well, then cover and cook over a medium heat for 10 minutes.
Add the chicken stock and bring to the boil.
Reduce the heat, cover and simmer for 20 minutes until the potatoes are tender.
Remove from the heat and blend in a food processor until smooth.
Return to the saucepan and gently hear through.
Add the sour cream and season to taste with salt and freshly ground black pepper.
Strain the olive oil and drizzle 5ml over each serving of soup.
Sprinkle with chives and serve immediately.