This is a lovely twist on the classic gratin dauphinois, in which layered potato and beetroot is gently cooked in garlicky cream. I prefer to use waxy potatoes as they keep their shape better but whatever you use, cut them as thinly as possible, ideally in a food processor or mandoline. The beetroot should be cut thicker because it has already been cooked.
Preheat the oven to 180°C/Gas 4. Rub the inside of a 26 × 18cm baking dish with the cut sides of the garlic, then grease it with butter.
Slice the potatoes very thinly. Cut the beetroot into slices about 5mm thick. Heat the cream gently over a medium heat until hot.
Layer one-third of the potatoes into the prepared dish (save the best-looking pieces for the top). Cover with half the beetroot slices and season with salt and pepper. Repeat before finishing with a last layer of neatly arranged potatoes and season again.
Pour the hot cream over the dish and bake for 1 hour, or until the potatoes are completely tender. If the top seems to be browning too quickly towards the end of the cooking time, cover it with foil. Serve warm.
HOW TO COOK BEETROOT: Wrap the beetroots and a good sprinkling of salt in foil and cook them at 180°C/Gas 4 for about an hour, or until they feel soft when pierced with a skewer. Leave them to cool a little before peeling.