Take a small sharp knife (pocket knives and utility knives work too) and cut an x into the flat side of each chestnut, piercing the shell.
Preheat the oven to 425 degrees Fahrenheit. Place a medium cold pot of water over medium heat and add the chestnuts. Bring the pot of water to a boil and cook for 2 minutes. Drain through a colander and then pat the chestnuts dry.
Place a large sheet of tin foil on a rimmed baking sheet. Empty the chestnuts onto the tin foil, keeping them in one flat layer towards the center of the tin foil sheet so you have several inches of space around them. Drizzle them with the melted butter and honey and sprinkle them with the sea salt and sage leaves. Place the rosemary sprigs on the foil as well, then fold the extra foil edges over the chestnuts and herbs, touching the edges of the foil together but leaving a little opening for steam to escape.
Place the pan in the oven and roast until the edges of the scored x mark start to curl up and the chestnuts are cooked through, about 30 to 45 minutes depending on the size of the chestnuts. Remove and allow to cool for 5 minutes before handling.
The nuts will still be very hot, but the skin and shell will come off easier while the nuts are still warm so you need to work quickly. To keep from burning yourself on the hot nuts, use a clean rag to hold each one. Pick one up from the pan, squeeze it slightly to help loosen the skin, then peel off the shell and skin. If you have a hard time getting the skin off, try squeezing the nut gently again, taking care not to squeeze it so hard that the nut crumbles. Repeat until all of the chestnuts are shelled and skinned. Drizzle with the pan drippings and diced fresh rosemary and toss to coat. Serve immediately.