Re celeriac: I have had it mashed, as fondant celeriac, roasted (as "chips" or with other roasted veg) and as a dauphinoise.
For all of them, you need to take off the outside "skim". I find it easiest to cut thick slices with a big knife and then use the big knife to chop off the edges.
Mash: cut into small dice and then boil until soft (surprisingly quick compared to neeps). Drain and mash (I use a potato ricer) with plenty of butter and/or olive oil.
Posh mash: as above and blitz (with cooked cauliflower/neeps if you want) in a food processor with an egg, Parmesan and sour cream/double cream/FF yoghurt.
Roasted celeriac: cut into chip shapes and then toss in oil (maybe chili oil, or some chili flakes or other herbs/spices). Roast until going crispy at the edges. For roasted veg, cut into chunks with other low carb veg like neeps, plus some peeled shallots and unpeeled garlic cloves and toss in oil.
Celeriac crisps: cut into very thin slices, coat in oil and roast. Check as they catch easily. Leave to try on kitchen paper.
Fondant celeriac: cut thick rounds of celeriac. Fry in oil until the outside is caramelised. Poach in chicken stock until al dente.
Celeriac dauphinoise: cut into thin slices, then layer in a buttered gratin/shallow dish with some finely chopped garlic. Pour over cream season with salt and pepper. Dot top with wee dods of butter. Bake for c1 hour at c150C - more or less depending on what else is in the oven.