Preheat the oven to 180°c (fan assisted)/390°f/gas 6.
Use a mandoline to slice the artichokes to a 3mm thickness. Place into a saucepan with the cream, milk, nutmeg and a good pinch of salt and black pepper. Stir well, then bring the pan to a boil. Reduce the heat to a simmer and cook for 10-15 minutes, or until the Jerusalem artichokes are fork-tender. If using, add truffle oil to taste. Add an extra pinch of salt too, if you think it needs it.
Meanwhile, heat a large frying pan over a low-medium heat and add a good glug of olive oil. Once hot, add the sliced onion and a good pinch of salt and cook gently for 10-15 minutes, stirring regularly and lowering the heat if they begin to catch, until the onion is soft and beginning to caramelise. Add the sugar and continue to cook for a further 5 minutes, until the onion is lovely and sticky.
Use a slotted spoon to spoon a third of the Jerusalem artichokes into a baking dish, leaving the cream mixture behind in the pan. Add half of the caramelised onions, a third of both the cheddar and gruyere and a good crack of black pepper. Spoon another third of the Jerusalem artichokes on top, along with the remaining caramelised onions, a third more of both the cheddar and gruyere and another good crack of black pepper. Finally spoon over the remaining Jerusalem artichokes, then pour over any of the cream mixture left in their pan.
Finish the gratin with the remaining cheddar and gruyere and a final crack of black pepper. Bake for 20-30 minutes, until golden and bubbling. It’s delicious with a lemony side salad, or as part of a larger spread.
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