@PenguinMum if cream is an issue there are 2 other recipes I use.
Boulangere Celeriac
Thinly sliced Celeriac again
Whole large onion thinly sliced and fried slowly in butter with garlic to taste. You do not want to colour it.
Oven proof dish and start with a layer of Celeriac slices. Add some of the soft onion mix then salt and pepper (not too much)
Sprinkle over some fresh thyme leaves.
Repeat as often as necessary.
Top layer is Celeriac slices.
Pour in some vegetable stock. I can't say how much as it depends how much Celeriac is in there. You want enough for the Celeriac to absorb all the stock (you can always add more stock)
Once the stock is in, grind more salt and pepper and add a few little bits of butter on top.
Into oven for about 1 hr Fan 160C
Lyonnaise Celeriac
This is similar to above but only needs about 30 mins in the oven (a bit hotter at 200C
Thinly sliced Celeriac and a large Thinly sliced onion again.
Once again soften the onion with crushed garlic to taste but in olive oil this time. You want to caramelise the onion this time to a nice golden brown.
Get some melted butter in a dish since, as you layer, you are going to coat the Celeriac slices in butter.
First layer buttered Celeriac (very messy hands with this)
Grind some salt and pepper on top then add a good amount of the onion mix
Guess what? Repeat and build up the layers about 3 layers.
Final one is buttered Celeriac and a grind of salt and pepper.
Into oven at 200C.
Check it after about 12 mins (I use the point of a knife). You want the Celeriac soft but with brown and crispy bits (especially the top).
You can Sprinkle with fresh chopped parsley before serving.