Separate head of garlic into individual cloves.
Toss in olive oil and wrap tightly in small piece of aluminum foil.
Bake in 350°F (180°C) oven for 45 minutes.
When the garlic has cooled to the touch you should be able to squeeze it out of the 'paper' shell of the individual cloves.
Mash the roasted garlic with a fork, or force through a fine strainer.
If you use a ricer as recommended below, simply rice the garlic along with the potatoes.
Peel and boil potatoes in salted water until tender.
Force the cooked potatoes through a ricer (recommended) or mash by your usual method.
Put the riced potatoes, garlic, sour cream, butter, and salt and white pepper in a mixer bowl, and whip at medium speed until smooth.
Add cream or milk to adjust consistency.