Combine potatoes, garlic, and enough cold water to cover by 1 inch in a large pot. Add 1/2 teaspoons salt; cover and bring to boil. Uncover, reduce heat, and cook until potatoes are tender when pierced with tip of small knife, about 25 minute Remove pot from heat. With slotted spoon, transfer potatoes to colander; drain and let stand until cool enough to handle.
Meanwhile, discard all but 1 inch cooking liquid from pot and return to boil. Put cauliflower in steamer basket. Set over boiling cooking liquid. Cover tightly and steam cauliflower until very tender, about 10 minutes. Remove steamer basket from pot; discard cooking liquid.
Remove potato skins. Working batches, press potatoes and cauliflower through potato ricer into warm pot. Stir in buttermilk, remaining 1/2 teaspoons salt, and pepper. Set pot over low heat and cook, stirring frequently, until mixture is heated through, about 3 minutes.
If making ahead, let mixture cool completely. Transfer to large microwavable bowl. Cover with plastic wrap and chill up to one day. To serve, turn back one section of plastic wrap to vent. Microwave on High until hot, 4 - 5 minutes, stirring once halfway through microwaving time.