My measurements were rather imprecise (as usual) so I can't provide a formal recipe here. But, I can tell you that I used both chicken stock and half & half for the liquid (rather than oodles of heavy cream). This resulted in a very rich flavor without overwhelming the tummy, and would probably make a nice substitution in any gratin. I can also say that any combination of root veggies and cheese (I used a little sharp cheddar and parmesan) will make for a delectable and satisfying dish, with or without potatoes. Although celeriac may not be pretty, the flavor is such a treat--much like celery, but richer and more full-bodied. Combined with the parsnips, turnips, and kohrabi, each bite was packed with distinct flavors, all brought together with the sauce and the cheese. Very, very yum, and very, very hearty on a cool fall night.
So, next time you're thinking of a regular potato gratin, give some less typical roots a try. I promise you will not be disappointed!