Put the cream garlic and thyme over a medium heat. Bring to a simmer, then remove from the heat and allow to infuse for 10 minutes.
Pass through a sieve and set aside, discarding the garlic and thyme.
Slice the potatoes and beetroot to about 2mm thick use a mandolin if you have one.Pour a thin layer of cream into the bottom of a fairly deep 2 litre oven proof dish.Layer half the potatoes on top of the cream, pour over more cream and season to taste. Add all the beetroot in 1 layer.Add more cream, season, then top with the remaining potatoes. Pour over the remaining cream and season.
Bake for 50-55 mins or until bubbling, golden and tender - a knife should go through without resistance.