cooking vegetables for mashing
After trying steaming a few times as renegade suggested, I've decided I prefer roasting:


As with steaming, everything stays in the vegetables, they're pretty easy to test, and they don't get waterlogged. 
It gives some nice caramelization, contributing a little to the end flavor.
It leaves them drier than steaming, which I like because it gives me the freedom to add plenty of other liquids (e.g. milk).
It's easy to roast large quantities of whole vegetables (which wouldn't fit in anything I have to steam in), so I can make large batches. And cooking them whole means there's less to sort through when pulling out ones that got done faster.


To each their own, but this works for me!