If using the garlic, start by preheating the oven to 425F. Peel the outer layer of skin around the garlic, leaving a layer of skin around each clove. Chop off 1/4 to 1/2 inch (5mm-1cm) from the top of the garlic. You should expose the top of each clove. Place the garlic in a piece of aluminum foil and drizzle with olive oil. Wrap the aluminum foil around the garlic and roast for 35 minutes at 425F. Once it's done roasting, unwrap the foil and let it cool until it's easy to handle. Set aside.
Meanwhile, peel your potatoes and cut each potato in half. Cut each half in another half (you should get 4 medium pieces from each potato). Transfer to the Instant Pot and cover with water, about 5 cups.
Set the Instant Pot to the "steam" setting for 10 minutes. Make sure the handle is set to "sealing".
Once the potatoes are done, turn off the Instant Pot and release the steam immediately.
Drain the potatoes and return them to the Instant Pot. They should be fork tender. Add the milk (start with 1/2 cup), salt and butter. If using the garlic, squeeze the garlic cloves from the skins and add it to the potatoes. Mash until creamy and fluffy. Add the remaining 1/2 cup of milk, more or less as needed to get your desired texture. Taste and adjust the salt and butter as needed. Serve immediately!