1. Heat the oven to 450°F. Don't prepare the celeriac until you want to use it or it will turn brown. Mix the garlic with the thyme leaves and leave until needed.
2. In a frying pan heat a small knob of butter and fry the mushrooms. Saute until liquid has evaporated. At this point, throw in the chopped garlic, salt to taste and thyme, and stir through the mushrooms, keeping on the heat for another minute or so. Put to one side.
3. Generously butter an ovenproof frying pan, about 15cm in width, and season with a little salt and pepper. Take the celeriac and cut away the skin. You will have to cut a little deeper than with other roots as the outside of this vegetable tends to be woody and fibrous. Spilt the celeriac in two. On a mandolin, or with a strong and sharp knife, slice each half of your celeriac crossways into thin slivers. Arrange half of the celeriac slices in an overlapping star shape around the bottom of the pan. Pour in your mushrooms and arrange them in the center. Pour over the cream. Arrange the rest of the celeriac slices over the top and dot with the remaining butter.
4. Place the pan on a burner turned to high. Wait until you hear some cooking sounds then leave the pan there for about 3 minutes. This ensures that what will be the top of the turned-out dish will be well browned. Put a hardy plate or lid that fits inside the edge of the pan over the top of your torte before cooking in the oven for 20 minutes. When cooked a skewer will push easily through the celeriac.
5. Remove the plate from the top and, with a spatula, run the tip round the inside of the pan to help the contents release easily when the dish is turned out. So now turn it out. Slice to serve.