2mediumleeks (white parts only)halved lengthwise, rinsed well, and cut into 3-inch pieces
1/2poundcarrotscut into thin 3-inch sticks
1tbsp.unsalted butteroptional, cut into small pieces
Coarse salt and ground pepper
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.