In actual fact, as far as side dishes for roast chicken go, this one is a definite keeper. I peeled the celeriac, halved and then sliced it. I popped the pieces in a pan of cold water along with a couple of chopped shallots and brought it all to the boil. Once drained, I layered the celeriac and shallot in a roasting pan with some sliced mushrooms, scattering a bit of chopped tarragon (mmmm...) between the layers. Poured over a 284ml carton of whipping cream (though double would also be fine, that's just what I had to hand), ground over some (white) pepper and covered it with a handful of grated parmesan. That was it; ready for 45 minutes in the oven at 180ºc (which was, helpfully, the second half of the chicken's cooking time). I allowed the gratin to rest for 10 minutes alongside the chicken after taking them out of the oven; I always think that baked dishes like this benefit from being served on the warm side, rather than searingly hot, they seem to taste more of themselves, somehow.