Preheat oven to 400 degrees F. Cut 1/4 inch off the pointed end of the head of garlic to expose cloves. Drizzle with a little olive oil; wrap in foil. Scrub potatoes and prick with fork. Place potatoes in preheated oven; bake 20 minutes. Add garlic. Continue baking potatoes and garlic 35 to 40 minutes or until potatoes are tender and garlic is soft and browned.
Holding hot potatoes with towel, split potatoes and scrape flesh into saucepan over low heat. Squeeze roasted head of garlic from the bottom to push cloves out of their papery skins and into potatoes. Add butter, sour cream, and cream cheese. Mash with potato masher. Season with 1 teaspoon salt and 1/4 teaspoon black pepper. Gradually stir or whisk in hot milk until potatoes are light and fluffy. Transfer to serving bowl. Top with Mushroom Sauce. Serves 8.