Slice about 1cm/ 1/2 inch off the bottom of your celeriac and roll it on to that flat edge, so its nice and safe to slice.
Slice and dice it all up into 1cm/ 1/2-inch-ish cubes.
Dont get your ruler out they dont have to be perfect.
Put a casserole-type pot on a high heat, add 3 good lugs of olive oil, then add the celeriac, thyme and garlic, with a little seasoning.
Stir around to coat and fry quite fast, giving a little colour, for 5 minutes.
Turn the heat down to a simmer, add the water or stock, place a lid on top and cook for around 25 minutes, until tender.
Season carefully to taste and stir around with a spoon to smash up the celeriac.
Some people like to keep it in cubes, some like to mash it, but I think it looks and tastes much better if you smash it, which is somewhere in the middle.
You can serve this with just about any meat you can think of.