This dessert is elegant enough to serve with Thanksgiving dinner, either as a pre-dessert course, or just as a lighter alternative to heavy pies and cakes.
Combine sugar with 1 3/4 cups water in a medium saucepan. Stir well, and cook the mixture over medium-high heat, stirring occasionally, until sugar has dissolved, 5 to 8 minutes. Transfer the syrup to a metal mixing bowl, and set bowl over an ice bath, or place in the refrigerator to chill, about 30 minutes.
Meanwhile, peel and core 3 pears. Chop into 1/4-inch dice and toss with 2 tablespoons lemon juice. Transfer to a medium saucepan. Add cranberries. Cover and cook over medium heat until the juices are released, 6 to 8 minutes. Reduce the heat to medium low and cook, covered, until pears are very soft, 12 to 18 minutes. Transfer mixture to a food processor and process until smooth. (At this stage, the puree may be passed through a fine strainer to get a smoother texture.) Transfer puree to metal bowl; let chill over an ice bath or in refrigerator, about 30 minutes.
Combine chilled puree with syrup and 1/4 cup lemon juice. Transfer the mixture to an ice-cream machine and freeze, following the manufacturer's instructions. If a machine is not available, place the mixture in an 11 by 6 by 2 3/4-inch plastic container (this size container works best) in the freezer for 1 hour. After 1 hour, stir with a fork. Continue to freeze, stirring every 30 minutes, until the sorbet has set and is completely frozen, about 4 hours.
Meanwhile, make the serving shells. Cut the top inch from the 6 remaining pears, and reserve the tops. Using a melon baller, scoop out as much flesh from the pears as possible, leaving the skin intact. Brush the insides of the pears with the remaining 2 tablespoons lemon juice. Place the pears and their tops in a large plastic container, and cover; transfer to the freezer for at least 2 hours. The shells may be prepared 2 to 3 days ahead. To serve, fill frozen shells with sorbet, and garnish with the pear tops. Serve immediately.