Bring 6 cups of water to a boil and carefully add the potatoes. Boil on medium to medium-low for 15 minutes or until you can easily pierce them with a knife.
Using a slotted spoon transfer the potatoes to a large heat-proof mixing bowl and add the butter, milk (or if using flour, add the roux), rosemary, and a little salt and white pepper. Mix on medium-high speed until fluffy, scraping sides as needed.
Taste and season with additional salt and pepper, to taste.
Serve immediately or store covered in a sealed leftover container and refrigerate for up to three days. Freezing is possible but it slightly changes the texture of the potatoes.
https://glutenfreerecipebox.com/rosemary-mashed-sweet-potatoes-flour/