Preheat oven to 425°F. Place a large sheet of foil on a rimmed baking sheet. Place chestnuts, flat side down, on a work surface. Using a utility knife or a sharp paring knife, carefully cut through the shell on the rounded side of each chestnut to score a slit or X. If not punctured in some way, the build-up of steam pressure within the shell will cause the nut to explode with considerable noise, force and mess.

Soak in a bowl of hot water for 1 minute (this helps them steam while roasting); drain chestnuts and pat dry.

Place in a medium bowl. Add rosemary, butter, salt, and nutmeg. Season with pepper and toss to thoroughly coat.

Arrange chestnuts in a single layer in the center of foil (a few might overlap) and gather up edges of foil around chestnuts, leaving a large opening on top.

Roast until the peel begins to curl up and chestnuts are cooked through, 30-45 minutes, depending on size and age of nuts.

Transfer chestnuts to a platter, using a spatula to scrape in any butter and spices with them, and toss to coat. Season with more salt, if desired. Serve hot or warm.