When cooking in parchment, choose vegetables with similar cooking times so they'll be ready all at once. Feel free to add fresh herbs or lemon, orange, or lime zest for added flavor.
Preheat oven to 400 degrees. Place potatoes, leeks, carrots, thyme, and butter (if using) in center of a 24-inch-long piece of parchment. Season with salt and pepper. Fold into a twist or envelope shape.
Place packet on a rimmed baking sheet and bake until packet is puffed up and potatoes are tender, 25 to 30 minutes.