This is a layered casserole. Try to slice the vegetables as thinly as possible, otherwise the vegetables will take longer to become tender. A mandolin or the slicing disc of a food processor are good bets, though the mandolin allows for the entire root bulb to be sliced. Otherwise, a sharp knife is your best ally.
Turnips and potatoes are good here, too. Be aware that using red beets will color the casserole, which is not a bad thing, necessarily. Besides, or in addition to dried herb(s), you can use spices, or spice mixtures, such as curries. Fresh herbs are also nice, thyme and rosemary especially.
some oil for greasing a baking dish
1 lb or so celeriac, peeled
1 lb or so chioggia beets, peeled if you like
2 t dried thyme, or other herb, or a combination of herbs (see headnote)
1 c broth or water
salt and pepper, to taste
1. Preheat oven to 375°F. Grease a suitable baking dish. Slice your chosen roots as thinly as possible. Layer, alternately, the roots, sprinkling each layer with some of the dried or fresh herb, and/or spice, plus some salt and pepper before add the next layer. Pour the broth evenly over the vegetables.
2. Cover baking dish with its lid or with foil and place in oven. Roast for half an hour. Remove lid or foil, increase heat to 400°F, and roast for about 10 minutes longer, until vegetables are tender and the top is lightly browned. Allow to sit for a few minutes before serving.