Slice about 1/2 inch off the bottom of your celeriac and roll it onto the flat edge, so it's safe to slice. Peel the outer skin off the celeriac, exposing the pearly white flesh. Slice and dice it all up into 1/2 inch-ish cubes.
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 salt and pepper. 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.