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, 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 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.