Preheat oven to 400 degrees F.
Roast the garlic by slicing off the top 1/2 inch of the entire head of garlic. Drizzle on a little extra-virgin olive oil. If using rosemary, place cleaned rosemary sprigs on top of the garlic and wrap the garlic and rosemary completely in foil. Roast at 400 degrees F for 30 minutes.
While garlic is roasting, prepare potatoes by washing them and removing any eyes or bad spots. Keep as much of the skin on as you prefer - or none at all if you like. Cut potatoes into uniform 1-2 inch chunks and place in a pan with enough water just to cover them.
Bring to a boil using med-high heat, then reduce to medium so that potatoes are simmered or boiled gently for about 20 minutes or until they are easily pierced with a fork.
Remove garlic from oven.
Drain the potatoes and return to the pan. Mash lightly with a potato masher.
Heat the milk and butter in the microwave, on high, for 30 seconds - or until warmed but not boiling or scalded. Pour it in with the potatoes.
Unwrap the garlic and squeeze the roasted garlic out of the skin and into the potatoes. If using rosemary, peel the rosemary from the stem and add whole to the potatoes or chop before adding, if you prefer.
Whip the potatoes using a hand mixer with a wire whip attachment. You can also use regular beaters, but the wire whip tends to produce a fluffier mashed potato. Whip to desired consistency.
Season with 1/2 tsp. kosher salt and fresh ground pepper. Makes about 7 c. (about 3/4 cup per serving)