1. Preheat the oven to 180°C/350°F/gas mark 6. Bring the milk, cream, thyme and garlic to the boil in a saucepan then remove from the heat and leave to infuse for 10 minutes. Season with plenty of salt and some black pepper.
2. Grease a 2 litre square Pyrex dish with butter then layer the celeriac and potato slices in the bottom. Pour over the milk and cream mixture and press down with a spatula to ensure the vegetables are completely covered. Dot the top with butter then bake for 1 hour until the top is golden and the vegetables are tender (during the first 30 minutes of cooking, press the vegetables down with the back of a spatula occasionally). Remove from the oven and stand for 10 minutes before serving.