Preheat oven to 425º. Cut off the very top of the garlic head, and drizzle with olive oil.
Place in a garlic roaster, muffin tin, or wrap in aluminum foil. Roast for 45 minutes or until the top of the bulb is oozing with caramelized garlic. As the garlic cools, place potatoes in an oven-proof pot of water and bring to a boil. Cook for 20 minutes or until potatoes are tender when stuck with a fork. Drain the water. Then smash the potatoes with a potato smasher.
Using a butter knife, remove the garlic cloves from each section of the bulb and place in pot with the smashed potatoes. Add margarine/butter , milk, and salt and pepper. Stir until you reach desired consistency. Add more milk if you want to make it smoother.
Reheat gently in the oven, after the turkey comes out and is resting.