The celeriac mash is fantastic with fish too.  You can serve the celeriac mash on its own or you can mash it together with potatoes, for a roast dinner I usually serve it on its own.

1 large celeriac, peeled and diced
1 onion, finely chopped
A thin slice of butter
Salt and pepper

Bring a pot of salted water to the boil, add the celeriac to the pot and cook until tender about 20 minutes.

In the meantime, melt the butter in a saucepan and sweat the onion until translucent but not coloured.  This will take about 5 – 7 minutes.

When the celeriac is cooked drain the water and put the pot back on a low heat for a minute so that any excess water evaporates.  Now add the onion to the pot together with some salt and pepper and mash well.  Taste and adjust the seasoning if necessary.  Sometimes I add a little more butter to the mash for a richer taste.