The objective: A nice, pink, juicy leg of lamb has a final temperature of about 120 degrees F. Use a shallow roasting pan or baking sheet with a fitted rack. Elevating the lamb slightly off the bottom of the roasting pan allows the hot air to better circulate around the meat as it cooks.
Preheat the oven to 350 degrees F.
Toast the spices: In a medium bowl, combine the cayenne, fennel seeds, dry mustard, paprika, red pepper flakes and cinnamon. Mix the spices and pour them out onto a baking sheet. Place the tray inside the oven for 1 minute to warm the spices and allow them to toast slightly. This will awaken their flavors. Remove the tray from the oven.
Make the paste to coat the leg of lamb: In a small bowl, combine the olive oil, molasses and salt. Add the toasted spices and mix together to create a paste. Coat all sides of the leg in the seasoning paste. If you have time, allow the lamb to sit out for about an hour and come closer to room temperature. Alternatively, you can marinate the lamb like this for several hours (or overnight) in the refrigerator before cooking.
Cook the lamb: Place the lamb leg in the center of the fitted rack on the roasting pan and place it in the center of the oven. Cook, undisturbed, for 30 minutes. Use an instant-read meat thermometer and take the temperature of the thickest part of the leg meat. Your meat thermometer should read about 85 degrees F. Cook for an additional 30 minutes. Ideally, the thickest part of the leg should come to about 118 degrees F. Remove the lamb from the roasting pan to a flat surface to rest.
Make the sauce: In a medium bowl, whisk together vinegar and mustard. Place the fresh chiles on a flat surface and, with a small knife, cut them both into thin rounds, ribs, seeds and all.
Pour off some of the drippings from the bottom of the roasting pan. Put the pan on top of a burner on your stove. Start a low heat under the pan and when the drippings begin to sizzle, add in the mustard, vinegar and fresh chiles. Taste for seasoning. Pour over the leg of lamb.
Carve the meat: Begin carving at the loin end of the leg; you want to carve at a right-angle relative to the grain of the meat. Observe that this meat has lines almost like a piece of wood, aim the blade of your knife at a right-angle to the grain and slice thin pieces. Arrange the slices on a platter with the sauce.