Preheat the oven to 350℉.
In a medium saucepan, combine the parsnips, turnips, garlic and milk. Make sure the milk is covering the vegetables (add more if necessary).
Bring to a simmer and cook until the vegetables are fork-tender, 15 to 20 minutes. Strain, reserving the cooking liquid.
Place the vegetables in a large bowl with the butter and mash, adding small amounts of reserved cooking liquid to get a thick mashed consistency. Stir in the parmesan and parsley.
Mist the cups of a muffin tin with cooking spray. Scoop the mashed vegetable mixture into the cups. Sprinkle the tops with black pepper and bake until golden brown, about 20 minutes.