Traditionally, potatoes are mashed with milk or cream, but don’t be afraid to try other liquids.
Since going dairy free, I have often used almond milk in mashed potatoes. My family didn’t notice any difference and loves them just as well as they did with cow’s milk.
Potatoes can also be mashed with broth. One of my kids loves red potatoes mashed with olive oil and chicken broth. Add freshly grated Parmesan cheese if you can eat it.