I like to add celeriac to mashed potatoes..it gives a good flavour to them. Works fine with either milk or cream.
I used to make Celery Root "Mashed Potatoes" myself, until my local Grocery Stores quit carrying Celery Root. I Miss it so much. It is creamier & tastier than Mashed Potatoes. I never tried it like this with 1/2 Potatoes. I used strictly Celery Root with white Pepper, salt, cream & Butter. I put it in my blender & My Family liked it better than Mashed Potatoes.....it is a little sweeter flavor than Potato.....a creamy, delicious taste. Does NOT taste like Celery! - 22 Nov 2010 (Review from Allrecipes USA and Canada)
The celeriac added a delicious, subtle celery flavor to the mashed potatoes. I used twice as many potatoes (6), less butter (2 T), and skim milk. Turned out delicious, but I would say it probably serves more like 4 than 12. - 08 Dec 2010 (Review from Allrecipes USA and Canada)
This was great! A tip: use rice milk instead of cream (you'll never know the difference) and heat it before adding it in. It will combine better and won't drop the temperature of the potatoes! - 20 Feb 2014 (Review from Allrecipes USA and Canada)