Make the pear mixture: in a kettle combine the cranberry juice, the sugar, the cinnamon stick, and the lemon juice, bring the mixture to a boil, and add the pears. Heat the mixture until it just comes to a simmer and simmer the pears gently for 10 to 15 minutes, or until they are just tender. Remove the kettle from the heat, stir in the cranberries, and let the mixture cool. Chill the mixture, covered, for at least 8 hours. The pear mixture may be made 2 days in advance and kept covered and chilled.
Make the crust:
In a food processor blend together well the flour, the brown sugar, the ginger, the cinnamon, the allspice, and the salt, add the butter, and blend the mixture until it resembles coarse meal. In a small bowl stir together the egg yolk and the molasses, add the mixture to the flour mixture, and pulse the motor, blending the mixture until it is combined well but still crumbly. Turn the mixture out into a 10inch tart pan with a fluted removable rim and press it onto the bottom and up the side of the pan. Chill the crust for 30 minutes. Prick the crust with a fork, line it with foil, and fill the foil with the rice. Bake the crust in the lower third of a preheated 375 degree oven for 15 minutes, remove the foil and rice carefully, and bake the crust for 10 minutes more. Let the crust cool in the pan on a rack.
Transfer the poached pears and half the cranberries with a slotted spoon to paper towels to drain, discard the cinnamon stick, and reserve the syrup with the remaining cranberries. In a small bowl whisk together the eggs, the sugar, the sour cream, the milk, the vanilla, the zest, and a pinch of salt, stir in the drained cranberries, and spoon half of the custard into the crust. Slice 4 of the pear halves crosswise on the diagonal, arrange them decoratively on the custard with the unsliced pear halves, and spoon the remaining custard around the pears. Bake the tart in the middle of a preheated 325 degree oven for 50 to 55 minutes, or until the custard is just set. Remove the rim of the pan and let the tart cool on a rack. The tart may be made up to this point 8 hours in advance, cooled completely, and chilled.
In a saucepan boil the reserved syrup and cranberries over moderately high heat until the syrup is reduced to about 1 cup and is jellylike in consistency and transfer the cranberries with a slotted spoon to a plate to cool. Brush the pears with some of the cranberry glaze and arrange the cranberries around the edge of the tart. Serve the tart warm or chilled