Step 1Heat the oven to 325 degrees. Pat the roast dry with a paper towel, then season all over with 1 tablespoon salt.
Step 2Grind the peppercorns, allspice berries, cumin seeds, dried mint, cinnamon and nutmeg in a coffee grinder or with a mortar and pestle. Spread the spice mixture onto a baking sheet and roll the roast in the spices until it is covered.
Step 3Heat a large braising pan over medium heat, then add the olive oil and heat until hot. Add the roast and sear it on all sides until browned. Remove the roast to a plate and set aside.
Step 4Add the onion and garlic to the pan and saute until the onion is translucent. Add the lamb back to the pan, along with any juices that have accumulated. Add the broth and pomegranate molasses, so that the liquid comes halfway up the roast. Place a piece of parchment paper over the roast and cover the pan with the lid or a tightly fitted piece of foil.
Step 5Braise in the oven for 2 hours. Halfway through cooking, turn the roast over so that the uncovered portion is submerged in the braising liquid. The finished roast will be well-done but fork-tender. Remove the roast to a plate and allow to rest, covered, for 10 to 15 minutes.
Step 6Make the sauce: Skim and discard the fat from the top of the braising liquid. Strain the liquid into a medium saucepan and bring it to a boil over high heat. Reduce by one-fourth or until desired consistency is achieved. Season with salt to taste. Slice the roast and serve with the sauce spooned over it.