Servings: 6 as a side

400ml double cream 400ml double cream
200ml whole milk 200ml whole milk
50g unsalted butter, plus 15g extra for greasing 50g unsalted butter, plus 15g extra for greasing
2 banana shallots, peeled and thinly sliced (100g) 2 banana shallots, peeled and thinly sliced (100g)
10g rosemary, leaves picked and finely chopped (6g) 10g rosemary, leaves picked and finely chopped (6g)
7 garlic cloves, peeled and thinly sliced 7 garlic cloves, peeled and thinly sliced
¾ tbsp black mustard seeds ¾ tbsp black mustard seeds
Salt and black pepper Salt and black pepper
800g king edward potatoes, peeled and cut into 1mm-thick slices (on a mandoline, ideally) 800g king edward potatoes, peeled and cut into 1mm-thick slices (on a mandoline, ideally)
150g salt-and-vinegar crisps (store-bought) 150g salt-and-vinegar crisps (store-bought)
100g gruyere, finely grated 100g gruyere, finely grated
10g chives, chopped finely 10g chives, chopped finely

Heat the oven to 190C (170C fan)/375F/gas 5. Put the first seven ingredients in a medium saucepan with a teaspoon and a half of salt and half a teaspoon of cracked black pepper. Put on a medium heat, bring to a simmer and cook gently for 15 minutes, until the shallots and garlic have softened. Keep on a low heat while you get on with the potatoes.
Grease a 24cm x 14cm rectangular baking dish with the extra butter and arrange a slightly overlapping single layer of sliced potato on top. Ladle over some of the warm cream mixture, then repeat the layers until the potatoes and cream mix are all used up. Cover tightly with foil and bake for an hour and a quarter, until the potatoes are tender.
While the potatoes are baking, put the crisps in a medium bowl and crush them roughly. Add half the grated gruyere and a good grind of black pepper, mix to combine and set aside.
Once the gratin’s time is up, lift off the foil and turn up the oven to 200C (180C fan)/390F/gas 6. Sprinkle the remaining 50g gruyere on top of the gratin, then scatter over the crisp and cheese mixture and return to the oven, uncovered, for another 15 minutes, until the cheese is bubbling and golden. Remove from the oven, leave to settle for 15 minutes, then sprinkle over the chives and serve warm.