Place the parsnips in a large pot. Cover with water and add a generous pinch of salt. Cover the pot and bring to a boil. Once boiling, remove the lid and lower the heat to a simmer. Boil the parsnips for about 15-20 minutes until they’re fork tender and easily pierced with a knife.
Reserve 1 cup of the cooking liquid then drain the parsnips. Return them to the pot. Add the roasted garlic cloves, minced rosemary, butter and milk. Mash with a potato masher until creamy and smooth adding a bit of the reserved cooking water as needed to get a nice creamy consistency. This can also be done in a food processor if preferred.
Season to taste with salt and pepper. Garnish with rosemary and an additional pat of butter, if desired, before serving.