Why do some traditional recipes call for simmering celery root in milk or milk/water?
According to Chow.com, "cooking the celery root in milk mellows the vegetable’s bite. However, if you prefer a more herbaceous flavor, substitute water for the milk."
If you want to gild the lily, use cream.
Interesting. I've never heard of cooking in milk or cream, and it isn't as if cooked celery root has a particularly strong taste. I suppose if you were then making a cream of celery root soup or something, it might be nice to have cooked in the milk ahead of time and then use that in the soup...
It's nice to do if you're making a puree - basically ensuring that you don't drain away the nutrients (+flavour) when you add cream/milk later on in the mashing stage. Also you can infuse bay, herbs, other spices into the liquid at the same time. Same process works well for cauliflower or parsnip puree.
Don't throw away leftover cream/milk - it's great for polenta!