Bearing asked for my roasted celeriac recipe, and I am posting it even though it is not much of a recipe: fat, salt, heat.
You want a moderately hot oven but you have some flexibility in case you're cooking something else at the same time: 375 or 400 is about right. (I am having a moment of nostalgia for the cookbooks of my grandmother's era, which would often enjoin the reader to put something in a slow oven (or a moderate oven, or a hot oven) rather than specifying a particular temperature. Probably I would not feel nostalgia for those days if I had suffered through fallen cakes or crunchy-in-the-middle baked potatoes after making a faulty guess at the right oven temperature, but oh well.)
Peel yourself as many root veggies as your eaters would like, bearing in mind that leftovers will be a treat. Mix it up: celeriac was especially nice last night mixed in with the potato and sweet potato bites, but rutabaga is a favorite of mine and roasted sunchokes, if you can find them, will make you burst into song. Chunk them up into roughly equal pieces; there's no need for precision measurement here. Spread them in a single layer in a foil-lined jelly roll pan. Drizzle generously with olive oil, making sure each piece gets a light coating. Sprinkle with salt and pepper and slide into the oven.
Peek at them after 20 minutes. You may want to flip them over for even browning. You may think that's too much hassle. Stick a fork experimentally into a fat chunk and see how much resistance it meets. I would estimate that you'll need 40 minutes for pieces on the bigger end of bite-sized, but ovens vary, as do vegetables.
An option I use sometimes is to parboil the veggies first. There's a simple method I use that comes from Crescent Dragonwagon's Passionate Vegetarian: parboil small but not teeny potatoes for 6 or 8 minutes and carrots for 4-6 minutes depending on size. (For celeriac I'd cut it into potato-sized pieces and treat like potato.) Rub them with olive oil and roast them for about 45-50 minutes, drizzling them with soy sauce in the middle of that cooking time. She calls these Roasty-Toasty Potatoes & Carrots; my copy of Passionate Vegetarian opens to that page.
There's a more complicated Nigella Lawson method that I use for special occasions and dream about in between. I've never made it with celeriac but I expect it would be fabulous. Cut 5 or 6 pounds of medium-sized potatoes in thirds and boil them for four minutes. Drain them and put them back in the pot, where you will sprinkle them with 2 T. flour, clap on the lid, and shake it like fury. This floury coating will give them a fabulous crisp crust. You can leave them at this stage for a while if you are cooking a fancy holiday dinner with multiple moving parts.
When you're ready for your potatoes to fulfill their glorious destiny, get your oven nice and hot. She says to crank it as high as it will go, but I wouldn't exceed 475, personally. I'd be doing battle with my smoke alarm the whole time if I cranked it up all the way. I usually divide them between two pans: a 9 x 13 pan with vegetarian potatoes cooked in olive oil, and an 11 x 15 pan for the meat-eaters (not a jelly roll pan, though; you need taller sides to prevent a conflagration), in which I use duck or goose fat. Heat the fat of your choosing in the pans until it is sizzling hot, and carefully tip in your potatoes. Nigella recommends 640g of goose fat for this recipe, which is a little less fat than you'd find in six sticks of butter. (I'd be cautious about using actual butter at that oven temperature, though. Seems like you could scorch the milk solids.) Cook for 25 minutes; turn and cook for 25 minutes more. The next time I make these I will keep some back to ensure that I have leftovers. You wouldn't think you'd need to worry about leftovers with 6 pounds of potatoes, but people will eat an awful lot of these.