1 celeriac (about 500g) peeled, quartered and thinly sliced
500g potatoes, peeled and very thinly sliced
Heat oven to 180C/160C fan/gas 4. Grill the bacon, if using, until cooked and lightly brown, then set aside.
Bring cream, milk, garlic, rosemary, chili and mustard to the boil in a medium saucepan, then turn off.
Pour a little of the cream mixture onto the bottom of an ovenproof gratin dish. Arrange a layer of celeriac, scatter with bacon, then season. Pour over some more of the cream mixture and repeat the same process, alternating potato and celeriac, finishing with a layer of potato. Cover with the remainder of cream mixture, then bake for 1-1¼ hours, until golden and vegetables are tender when a knife is stuck in. Leave to sit for 5 minutes, then serve.