Cut off all the outside bit of the celeriac, then chop into small 1cm or so squares. Actually, get someone else to do this - it's kinda like hard work!
Melt butter in skillet and throw in the thyme for about 10 seconds then add the chopped celeriac.
Pour over the olive oil, give it a stir and then cover, cooking on a low heat for about 15 minutes or so.
Check and stir it every once in a while so it doesn't stick. It'll start to brown and caramelise.
Remove the lid for the last 5 minutes and stir often and let the stuff brown.