Potatoes, Leeks, and Carrots in Parchment

INGREDIENTS

• 3/4 pound small new potatoes, halved or quartered if large
• 2 medium leeks (white parts only), halved lengthwise, rinsed well, and cut into 3-inch pieces
• 1/2 pound small carrots, halved crosswise if long, or large carrots, cut into thin 3-inch sticks
• 10 sprigs thyme
• 1 tablespoon unsalted butter (optional), cut into small pieces
• Coarse salt and ground pepper