Finely dice your onions. Peel potatoes and celery root.Peel and slice the potatoes very thinly. Grate the celery. Lay the potato slices and celery on a clean tea towel and pat dry.
Spread the onion, potatoes and celery in a gratin dish.
Preheat the oven to 350°F. Crush garlic. In a saucepan, slowly heat the milk and vegetable broth. Add garlic and salt and let it reach boiling point. Remove from heat and pour into the gratin dish. Cover with tin foil and bake in oven for an hour. Remove tin foil and keep in oven for another 30 minutes.