Celeriac and Potato au Gratin is a useful side as it's simple to prepare ahead and then put in the oven to heat through. The principal is the same as any other vegetable gratin- thin slices of root veg, in a thick sauce, so that the veg cooks through and binds into a gooey, gorgeous chunk with a crispy top.
What you'll need to serve 3-5
1 Celeriac root
2 large, waxy Potatoes
1 small Onion, halved
Peppercorns for seasoning
150g hard Cheese, grated
Starting with the vegetables, peel your celeriac and scrub the potatoes.
Then slice to about 2cm thick- I'm using a mandoline for this, to get even, thin slices, but it's easy enough to do with a sharp knife. Once you have some slices, start layering them in a greased dish, half and half celeriac and potato. Don't worry about overlap, the holes will allow the sauce to permeate.
Whilst you're doing this, you can begin to prepare the sauce. A classic gratin dauphinois uses potatoes and a cheese sauce, but although we're using potato to bulk this dish out, the focus should be on the celeriac, which has a slightly more delicate flavour. To bring it out, use this, simpler sauce.
First, gently heat a pint of milk with the bay leaf, some peppercorns, and half an onion at a low temperature.
After about fifteen minutes, and once you've finished slicing and layering the root vegetables, strain this into a saucepan, then drop in a couple of small chunks of butter which have been rolled in flour.
Then slightly increase the temperature to thicken the sauce, being careful to stir constantly and scrape the bottom of the pan to prevent lumps. Once it's reached your desired consistency, after about five minutes, take it off the heat, ready to be poured over the sliced roots.
The crispy top is what defines cooking "au gratin", but this dish is already so starchy that breadcrumbs would be overkill, so just add a thin layer of grated cheese.
This prepared Gratin will keep covered in the fridge for up to a day until you're ready to put it in the oven, with the pot roast in this case, for 30 minutes at 240ºC/Gas Mark 7, or until the top is nicely browned.
So, here we have the meaty meal as a whole. The basis of this gratin can be used for any type of root vegetable or squash- bear in mind that aubergine or marrow is more delicate and would lose its shape if treated roughly. Hope you enjoy it- don't hesitate to let us know via email@example.com if you do! And if you don't, then why not vent on the discussion board at Facebook?