Place the potatoes, skin still on, in a pot and cover with cold water. Place over medium heat and bring to a simmer. Cook until they can be easily pierced with a knife.
In another pot, bring salted water up to boil. Add turnips and bring water down to a simmer. Cook until tender, remove from water.
In a 3rd and final pot bring cream or milk, garlic, and butter to a simmer for about 20 min then remove garlic from liquid.
With a paring knife, carefully peel the hot potatoes, cut them into small chunks and pass through a ricer or food mill. Next pass the turnips through.
Add the smashed root vegetables to your stand mixing bowl and turn on to a low setting, or do by hand using a large wooden spoon or spatula. Slowly add your warm cream/ butter liquid to the potatoes. Turn the mixer on medium high speed, or turn your arm up and whip to desired consistency. For a chunkier texture, you can also just use a handheld potato masher. Finish seasoning with kosher salt and white pepper to taste (start with just a 1/2 tsp and build from there.)