The sweetness of the pears balances nicely with the tart cranberries.
Peel pears, sliced in half lengthwise, and core. Squeeze lemon over pears; set aside.
In a heavy skillet over medium heat, melt butter. Add sugar and stir constantly until dissolved. Cook, swirling pan, until mixture turns a deep amber. Place pears in caramel and cook over medium heat for 20 minutes, turning occasionally, until soft.
Fit 2 pear halves, cut side up, in each tart mold. Fill in gaps with cranberries. Pour remaining caramel over fruit. Let cool.
Roll out pastry to 1/8 inch thick; cut out 4 4 1/2-inch circles. Lay one over each mold, tucking in all around. Chill 1/2 hour.
Heat oven to 400 degrees. Set tart molds on a baking sheet and bake 20 to 25 minutes, or until pastry is golden brown. Let cool 10 minutes. Place a dessert plate on top of each tart, quickly invert, and serve with creme fraiche or whipped cream.
You will need 4 4-inch ceramic tart molds.