Step 1

Use potatoes that are no larger than 1 1/2 inches in diameter.

Step 2

Adjust oven rack to lowest position and heat oven to 400 degrees. Toss potatoes, rosemary, salt, thyme, and pepper in large bowl until potatoes are well coated.

Step 3

Line baking sheet with 16 by 12-inch sheet of aluminum foil. Spread potato mixture evenly over foil, leaving 1 1/2-inch border. Flip potatoes cut sides down. Scatter butter and garlic over potatoes. Place second 16 by 12-inch sheet of foil over potatoes. Beginning at 1 corner, fold foil inward in 1/2-inch increments 2 to 3 times to seal edge. Continue folding around perimeter of foil to create sealed packet.

Step 4

Transfer sheet to oven and bake until potatoes are tender, about 40 minutes. Let potatoes cool for 5 minutes. Using tongs, tear away top sheet of foil, being careful of escaping steam. Serve.