Put the halved unpeeled garlic heads into a small roasting tin, drizzle with 1 teaspoon of the oil and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Meanwhile, heat the remaining oil and the butter in a saucepan, add the celeriac and onion, cover and fry gently for 10 minutes, shaking the pan from time to time.
Take the garlic out of its paper skins with the tip of a small knife and add to the celeriac. Pour in the stock, add a little seasoning and bring to the boil. Cover and simmer for 30 minutes, or until the celeriac is tender. Cool slightly.
Purée the soup in batches in a food processor or blender then pour back into the pan. Cool completely then chill until required.
When ready to serve, stir the milk into the soup and reheat until piping hot. Dry fry the pancetta until crisp and golden and cut into long thin strips. Stir half the cream into the soup then ladle into serving bowls. Swirl the remaining cream into the soup. Sprinkle the soup with the pancetta and serve immediately.