Bring a small saucepan of water to boil over high heat. Add the garlic cloves and boil for 2 minutes, then drain and peel.
In a heavy frying pan, melt half of the butter over low heat. Add the blanched garlic cloves, then cover and cook gently for 20-25 minutes, until very tender and just golden, shaking the pan and stirring occasionally. Do not allow the garlic to scorch or turn brown.
Remove the pan from the heat and cool slightly. Spoon the garlic and any butter from the pan into a blender or food processor fitted with a metal blade and process until smooth. Transfer to a small bowl, press plastic wrap onto the surface to prevent a skin from forming and set aside.
Peel and quarter the potatoes and chop the celeriac. Place in a large saucepan and add enough cold water to just cover them. Salt the water generously and bring to a boil over high heat.
Cook the potatoes until tender, then drain and work through a food mill or press through a sieve back into the saucepan. Return the pan to medium heat and, using a wooden spoon, stir the potatoes for 1-2 minutes to dry them out completely. Remove from the heat.
Warm the milk over medium-high heat until bubbles from around the edge. Gradually beat the milk, remaining butter and reserved garlic puree into the potatoes and celeriac, then season with salt, if needed, and white pepper.
Makes 6-8 From November 1, 2009 Fox & Fawn Farm Newsletter