Melt the butter in a pan. Sauté garlic, onions, mushrooms and celeriac for about five minutes until soft. Set aside.
Thinly brush a 23 x 33 centimeter (for 8 servings) ovenproof dish with oil. Spread half of the bread cubes evenly in it. Then, in layers, pour in half of the sausage, cheese, and vegetable mixture, and sprinkle the scallions on top. Add the remaining ingredients in the same way, layer by layer.
In a small baking bowl, stir together the mustard, milk, eggs and spices and pour over the ingredients in the gratin dish. Cover and refrigerate overnight to allow the bread to absorb the liquid.
Cook in the oven at 180 °C for about an hour.
Tip: Always use aromatic spices to refine your dishes!