In a large stockpot, cover 3 pounds of peeled, cubed russet potatoes with water. Bring to a boil, reduce heat and simmer until tender, about 15 minutes. Drain potatoes and return to pot. In a small saucepan, cover 6 separated and peeled garlic cloves with water. Bring to a boil, reduce heat and simmer until tender, about 10 minutes. Drain. In a food processor or blender, combine the garlic and 1/2 cup fat-free milk. Puree until smooth. Add pureed garlic and 1 tablespoon margarine free of trans fat to potatoes. Mash, season with black pepper and garnish with fresh parsley.