Peel and cube the celeriac into 1/2 inch pieces. Keep the cut vegetable in water to avoid discoloration.
Heat the olive oil in a medium sized pot over medium high heat. Fry the garlic for a minute or until fragrant. Drain the celeriac from the water and add to the pot. Sauté for a few minutes. Add the salt, pepper and sprigs of thyme and add just enough water to cover the vegetables. Bring to a gentle simmer and leave to cook for 25 minutes. After 25 minutes, the celeriac should be fully cooked (cubes are easily pierced through with a knife).
Towards the end of the cooking of the celeriac, warm the butter and cream together in a small pot over low heat. Do not boil. You just want to melt the butter and warm the cream.
Using a slotted spoon, transfer the cooked celeriac to your blender. Remove the sprigs of thyme. Don’t worry about any thyme leaves that have fallen off. Pour in the warmed butter and cream and with the lid firmly on, blitz until you have a smooth purée. Unlike potatoes, celeriac purées very well in a blender as it does not have the same starch as potatoes. You may need to stir the celeriac in the blender in between blitzing to make sure it all gets puréed. Don’t add any liquid until you are sure it is necessary. Taste the purée and add more salt or pepper if desired.
Transfer the purée to your serving dish and top with a drizzle of olive oil and some fresh thyme leaves for garnish. Enjoy warm.
This dish is great made a day in advance. Store, tightly covered with cling film, in the fridge.