Preheat the oven to 220°C (200°C fan)/425°F (400°F fan)/gas mark 7 (gas mark 6 fan). Fill and boil a kettle and get all your ingredients and equipment together.
Peel the beetroots and use a food processor or a mandolin to finely slice them. Scatter them in a deep baking dish approximately 20 cm × 25 cm. Slice the potatoes in the same way and scatter them over the beetroots. Shred the radicchio as you would a lettuce – avoiding and discarding the root – and add this to the tray too.
Pick the leaves off the stalks of the thyme and sage, slice the garlic and add to the dish, then grate over the zest of the lemon and season well with salt and pepper. Toss everything together, then use your hands to press and push everything level in the dish. Pour over the white wine and the hot stock – the liquid should come about halfway up the potatoes – and bake in the oven for 30 minutes.
Meanwhile, make the gremolata. Finely chop the parsley, garlic and chilli and put into a bowl with the orange zest. Mix well, season with a little salt and pepper and put to one side.
Once the gratin is browned on top and crisp around the edges, it should be ready; check by pulling a potato from the middle of the gratin and making sure it’s cooked through. Serve with a watercress or rocket salad and, if you like, some bread for mopping up the juices.