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Heat oven to 180C/160C fan/gas 4. Grill the bacon, if using, until cooked and lightly brown, then set aside.
Bring the cream, milk, garlic, rosemary, chilli 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 the cream mixture, then bake for 1-1¼ hrs, until golden and vegetables are tender when a knife is stuck in. Leave to sit for 5 mins, then serve.