Melt the butter in a large saucepan, add the onion and smoked garlic and cook over a medium heat for 3–4 minutes until softened. Stir in the potatoes, cover and cook for 5 minutes.
Add the stock and season with the smoked sea salt and pepper. Bring to the boil, then reduce the heat, cover and simmer for 30 minutes until the potatoes are tender.
Transfer to a food processor or blender, in batches, and blend until smooth. Return to the pan, stir in the milk and herbs and reheat gently.
Serve in warmed bowls with a spoonful of Greek yogurt, garnished with chives and freshly ground black pepper.