Cut small cauliflower in quarters; if large, use a heaping cup of florets. If using celery root, knife-peel then quarter; same with any other vegetable you'd like to add, like rutabaga etc..

Put all in deep saucepan: potatoes, cauliflower or celery root, thyme spring, milk, and water to just cover (add additional milk/cream/water if needed but try to keep the 50/50 ratio for best results).

Bring to boil and lower heat to a simmer; simmer 5 or 6 minutes covered then uncovered for 20 minutes until vegetables are fork-tender.

Using a potato ricer set over a warmed serving bowl, rice the potatoes and cauliflower/celery root. Add butter and stir to melt and combine, adding spoonfuls of the cooking liquid to get desired consistency.

Hold mashed potatoes in warm serving bowl covered with foil.