Place potatoes in a large pot and add water to cover by one inch. Generously salt the water, bring to a boil, and boil for about 20 minutes, or until potatoes are tender. Drain.
Preheat oven to 350 degrees.
Fill a potato ricer with the potatoes. Over a large bowl, compress the lever, pushing the smoothed potatoes through. Repeat until they are all riced (if you do not have a ricer, the potatoes can be mashed).
Mix in the remaining ingredients, beating the potatoes with an electric mixer until smooth. Place the potatoes in a greased 2-quart casserole dish and bake for 30 minutes, or until potatoes are hot in the center.
This dish can be refrigerated for one week or frozen.
Note: To roast the garlic, cut off the stem and the top part of the individual cloves (this exposes all the cloves). Place the head, cut side up, in a roasting pan. Drizzle olive oil over the cut part of the garlic. Bake at 375 degrees for 30 to 35 minutes, until the garlic softens. Squeeze the cloves out of their skins while still warm and run through a garlic press.