Preheat the oven to 190°C/fan 170°C/gas mark 5. Place the milk, garlic, thyme sprigs, bay leaf, nutmeg and shallot in a large saucepan and bring to a simmer. Turn off the heat and leave to infuse for 10 minutes, then strain and discard the garlic, shallot, thyme and bay leaf.
Add the celeriac to the pan with the milk and bring to a simmer. Cook for 10 minutes, until slightly softened, then carefully transfer the celeriac slices with a slotted spoon to a 23x23cm baking dish.
Mix the crème fraîche and chopped thyme leaves with the infused milk mixture. Pour this over the celeriac and bake for 40 50 minutes, until golden brown on top and soft when inserted with a skewer. Cover it with foil if it begins to brown too much.