Yes!  You can peel and cut the parsnips and parboil them up to 4 hours in advance and leave them in a bowl of water in the fridge.  You can even pre-roast the parsnips in advance but if you do that then don’t add the maple syrup and mustard, just roast for 20 minutes in the oil.  Then you can freeze the roasted parsnips and defrost overnight, then roast for another 20 minutes with the maple syrup and mustard added.