Line baking sheet with parchment paper; coat parchment paper with cooking spray.
Preheat oven to 350°. Stir together plums and next 4 ingredients in a large bowl. Let stand 30 minutes, stirring occasionally.
Unroll piecrust on prepared baking sheet. Roll into a 12-inch circle.
Drain plum mixture, reserving liquid. Toss plums in flour.
Mound plums in center of piecrust, leaving a 3-inch border. Fold piecrust border up and over plums, pleating as you go, leaving an opening about 5 inches wide in center.
Stir together egg and 1 Tbsp. water. Brush piecrust with egg mixture, and sprinkle with 1 Tbsp. sugar.
Bake at 350° for 45 minutes or until filling is bubbly and crust is golden. Carefully transfer tart on parchment paper to a wire rack; cool 20 minutes.
Meanwhile, bring reserved plum liquid to a boil in a small saucepan over medium heat. Boil 1 to 2 minutes or until slightly thickened. Let cool slightly. Brush or drizzle 1 to 2 Tbsp. hot plum liquid over exposed fruit in center of tart. Serve immediately with remaining plum syrup, and, if desired, Sweet Cream Topping.
Sweet Cream Topping: Stir together 1/2 cup sour cream and 2 tsp. brown sugar. Cover and chill 2 hours before serving. Stir just before serving.
Peach-Nectarine Tart: White peaches won't work as well in this tart because of their delicate texture. Prepare recipe as directed, substituting 3/4 lb. peeled nectarines and 3/4 lb. peeled peaches for plums and peach preserves for plum preserves.
Apricot-Almond Tart: Prepare recipe as directed, substituting fresh apricots for plums and apricot preserves for plum preserves. Omit allspice.
Fresh Cherry Tart: Prepare recipe as directed, substituting 1 1/2 lb. fresh cherries, pitted, for plums and cherry preserves for plum preserves.