Split the pears in half and cut out the core. Brush the flesh with 2 tablespoons olive oil and place face down on a hot grill. Wrap the garlic and shallots in an aluminum foil pouch, pour in 2 tablespoons olive oil along with a little bit of water to create steam and seal. Place the packets on the hot grill and roast until soft, about 15 minutes. In a saucepan, combine remaining oil, chicken broth, rosemary and vinegar. Add the grilled pears, roasted garlic and shallots to the saucepan. Cook until the mixture reduces by half. Transfer the sauce to a blender and puree until smooth. Serve with Leg of Lamb.