Potato gratin recipe
 For this rich and creamy potato gratin, the potatoes are par-boiled in the cream mixture, which allows their starch to help thicken the sauce. Adding bacon makes this a great main meal – serve with a sharply dressed leafy salad. 600ml/20fl oz semi-skimmed milk2 garlic cloves, bashed3 bay leaves, plus extra to garnish2 thyme sprigs, plus extra to garnish1 leek, thinly sliced400ml/14fl oz double cream900g/2lb waxy potatoes, scrubbed or peeled and thinly sliced (ideally on a mandolin)6–8 rashers dry-cured, smoked streaky bacon, snipped into small pieces with scissors (optional)butter, for greasing50g/1¾oz Parmesan, finely grated50g/1¾oz Gruyère, gratedsalt and freshly ground black pepper 600ml/20fl oz semi-skimmed milk 2 garlic cloves, bashed 3 bay leaves, plus extra to garnish 2 thyme sprigs, plus extra to garnish 1 leek, thinly sliced 400ml/14fl oz double cream 900g/2lb waxy potatoes, scrubbed or peeled and thinly sliced (ideally on a mandolin) 6–8 rashers dry-cured, smoked streaky bacon, snipped into small pieces with scissors (optional) butter, for greasing 50g/1¾oz Parmesan, finely grated 50g/1¾oz Gruyère, grated salt and freshly ground black pepper Method Put the milk, garlic, bay leaves, thyme and leek into a pan, bring to the boil, then turn off the heat and leave to infuse for 1 hour. Strain the milk and discard the veg and herbs. Put the flavoured milk in a large saucepan, add the cream and tip in the potatoes and 1½ teaspoons of salt. Bring up to a simmer, stirring occasionally; as soon as the cream starts bubbling, immediately take the pan off the heat. Season with pepper.If using bacon, cook in a frying pan over a medium heat until golden and crisp, then drain and set aside. Grease a large, shallow baking dish with the butter. Use a slotted spoon to layer roughly half of the potatoes into the dish, then spoon over some of the cream mixture. Mix the cheeses together and sprinkle half over the potatoes, adding the bacon, if using. Cover with the remaining potatoes, pour over the remaining cream mixture and scatter with the remaining cheese. You can chill the dish overnight at this stage (see recipe tip).Preheat the oven to 180C/160C Fan/Gas 4. Cover the dish with foil and bake for 1 hour (add an extra 15 minutes if fridge-cold). Remove the foil, scatter with extra bay leaves and thyme plus a grind of pepper, then bake for 30 minutes, until the top is golden and the potatoes are tender. Put the milk, garlic, bay leaves, thyme and leek into a pan, bring to the boil, then turn off the heat and leave to infuse for 1 hour. Strain the milk and discard the veg and herbs. Put the milk, garlic, bay leaves, thyme and leek into a pan, bring to the boil, then turn off the heat and leave to infuse for 1 hour. Strain the milk and discard the veg and herbs. Put the flavoured milk in a large saucepan, add the cream and tip in the potatoes and 1½ teaspoons of salt. Bring up to a simmer, stirring occasionally; as soon as the cream starts bubbling, immediately take the pan off the heat. Season with pepper. Put the flavoured milk in a large saucepan, add the cream and tip in the potatoes and 1½ teaspoons of salt. Bring up to a simmer, stirring occasionally; as soon as the cream starts bubbling, immediately take the pan off the heat. Season with pepper. If using bacon, cook in a frying pan over a medium heat until golden and crisp, then drain and set aside. If using bacon, cook in a frying pan over a medium heat until golden and crisp, then drain and set aside. Grease a large, shallow baking dish with the butter. Use a slotted spoon to layer roughly half of the potatoes into the dish, then spoon over some of the cream mixture. Mix the cheeses together and sprinkle half over the potatoes, adding the bacon, if using. Cover with the remaining potatoes, pour over the remaining cream mixture and scatter with the remaining cheese. You can chill the dish overnight at this stage (see recipe tip). Grease a large, shallow baking dish with the butter. Use a slotted spoon to layer roughly half of the potatoes into the dish, then spoon over some of the cream mixture. Mix the cheeses together and sprinkle half over the potatoes, adding the bacon, if using. Cover with the remaining potatoes, pour over the remaining cream mixture and scatter with the remaining cheese. You can chill the dish overnight at this stage (see recipe tip). Preheat the oven to 180C/160C Fan/Gas 4. Cover the dish with foil and bake for 1 hour (add an extra 15 minutes if fridge-cold). Preheat the oven to 180C/160C Fan/Gas 4. Cover the dish with foil and bake for 1 hour (add an extra 15 minutes if fridge-cold). Remove the foil, scatter with extra bay leaves and thyme plus a grind of pepper, then bake for 30 minutes, until the top is golden and the potatoes are tender. Remove the foil, scatter with extra bay leaves and thyme plus a grind of pepper, then bake for 30 minutes, until the top is golden and the potatoes are tender. Recipe tips You can assemble the gratin a day ahead (up to the end of step 4) and chill overnight. If you want to serve this with a roast and you need to roast your joint at a higher temperature (190–200C/170–180C Fan/Gas 5–6), you can prepare the gratin to the end of step 5 the day before. Chill overnight, then proceed with step 6 at the higher temperature.