Ingredients

Step by step

1. Put 1 tablespoon of the butter in a large pan with the olive oil; heat gently. Add the onions; season and fry very slowly for 40-50 minutes until very soft and golden, but not burnt. Stir regularly to prevent them browning too much, and add a splash of water if they start to stick to the bottom.
2. Add the thyme, beef stock, bay leaves and Marmite or Bovril to the caramelised onions. Bring to the boil and add more salt and the sugar if you need to. Simmer very gently while you prepare the potatoes. Heat the oven to 180°C, fan 160°C, gas 4.
3. Peel the potatoes; thinly slice using a mandoline or a food processor with a slicing disc. Grease a 20 x 30cm baking dish with butter.
4. Add a layer of potatoes to the base, overlapping them slightly, then using a slotted spoon, scoop out some onions from the stock, and scatter a thin layer on top (discard the thyme sprigs). Season each layer as you go, as the potatoes absorb a lot during cooking. Repeat until everything has been used up, finishing on a layer of potatoes.
5. Pour over the stock, don’t worry if there are some remaining onions, too. Season the top, then dot with the rest of the butter. Decorate with another sprig or two of thyme if you like.
6. Bake uncovered for about 1 hour 15 minutes until the top is crisp, and a knife easily goes through the layered potatoes. Leave to rest for 5-10 minutes before serving with a roast or grilled lamb chops.