Heat the olive oil and cook the onions until golden, about 10 minutes; if the onions stick, add some water Add the garlic, celery and celery salt and wine and cook until wine has almost entirely evaporated; add the tomatoes and their juices and broth and simmer for 5 minutes to bring the flavors together and thicken slightly.
Parboil the potatoes for 5 minutes; drain and pat dry; parboil celery root for 3 minutes; drain and pat dry.
Preheat the oven to 375 degrees. Lightly oil a shallow wide baking dish (gratin dish) which will accommodate the 2 layers of potatoes and celery root.
Spread a third of the tomato mixture in the bottom of the dish and top with alternating slices of parboiled celery root and potatoes; season carefully with salt and pepper. Top with tomato sauce, then layer potatoes and celery root and cover with sauce.
Cover dish and bake for 45 minutes or until ingredients are completely tender. Serve hot, warm or at room temperature.