Preheat the oven to 180 degrees Celsius (360 Fahrenheit).
In a pan, add the bacon lardons and cook for about 5 minutes or until crispy.
Transfer the lardons to a plate and set aside.
Wash and chop the leeks, and add them to the same pan, and sautee them in the bacon fat for 5-10 minutes or until tender. Add a bit of butter if there is not enough fat left from the bacon.
Add the milk and half of the cream, bring to a boil, then remove from the heat.
In a baking dish, add one third of the leek mixture, top with a third of the bacon, plus a third of the cheese.
Peel and slice the celeriac, then arrange half of the slices on top.
Repeat with more leeks, bacon, cheese and celeriac, then top with the remaining cheese and the cream.
Bake for 60 minutes or until golden, and the celeriac is tender.
Garnish with freshly chopped parsley.