Peel potatoes only if you wish. Cut potatoes into quarters. Place in a large saucepan and cover with water. Peel garlic. Add whole garlic to potatoes. Bring to a boil over high heat. Then reduce heat to medium. Boil, partially covered, until potatoes are fork-tender, 25 to 30 minutes. Meanwhile, thinly slice green onions. In a measuring cup, stir buttermilk with Dijon, salt and nutmeg. Drain potatoes and garlic. Return to saucepan and place back on hot burner. Gently stir until any water evaporates. Turn burner off. Mash potatoes and garlic with a potato masher. Stir in buttermilk mixture and green onions. Taste and add more seasoning, if needed.