It takes ages to cook, but should go soft like any root vegetable when cooked properly.
You can boil it in water, and then mash it like potatoes with a bit of salt, pepper and olive oil.(You might have to squeeze out some of the water by putting the mash in a cloth or sieve to drain for a while unless you like it sloppy)
I think Orka said she eats it in little raw pieces on salad like you would eat radishes.
Roast / saute it as Able suggests. (it goes really sweet then and is yum)
Roast really thin lighted salted slices on a low heat for several hours to make crisps / potato chips.
Or my personal favourite, grate some of it, along with an onion and a courgette (zucchini) into a mixing bowl, mix in salt, pepper and an egg, as well as a bit of buckwheat flour if you’re using it, and fry it in a pan a spoonful at a time like little pancakes. Yum! I have these with an avocado chopped up on top and some pureed garlic and caulifower sauce.