You can simply boil the leeks with either of the vegetables, but roasting them adds lots of flavor, especially if you deglaze the pan with lemon juice.

Heat a pot of salted water to a boil.

Heat the oven to 425 degrees.

Peel 1 lb. parsnips and boil, whole, until easily pierced with a knife.  Remove from the water and rinse to cool.

Peel 2 lbs. of potatoes (or leave unpeeled) and add to the water.  Boil until they are soft. Remove from the pot, reserving the water.

While the root vegetables are cooking, clean and dice 1 leek.  Toss in a cast iron skillet or baking dish with 2 T. olive oil and salt.  Roast until they are nicely browned but not burnt, 10-15 minutes.  Remove from the oven and drizzle with the juice of 1 lemon, then use a wooden spatula to deglaze the pan.

Slice the cooled parsnips in half lengthwise.  If they have a tough core, remove it and discard.  Roughly chop the tender parts of the roots.

Combine the leeks and lemon juice in a bowl with the root vegetables.  Mash, adding salt, pepper and butter (or more olive oil) to taste.  If you need more liquid, use a bit of the cooking water.  Don’t overmash, or they will be gluey.