Combine the first 4 ingredients in a large bowl, and beat at medium speed of a mixer for 2 minutes or until mixture is light and fluffy.
Add 1 egg, and beat at high speed for 1 minute or until mixture is very smooth. Lightly spoon the flour into dry measuring cups, and level with a knife.
Combine flour and salt, and add to sugar mixture, beating at low speed just until the flour mixture is moist. Gently shape the dough into a ball on heavy-duty plastic wrap, and cover with additional plastic wrap. Freeze dough 2 hours.
Preheat oven to 40
Roll dough, still covered, into a 14-inch circle on a lightly floured surface.
Remove bottom sheet of plastic wrap.
Place the dough on a large baking sheet coated with cooking spray, and remove top sheet of plastic wrap. Arrange plum slices over dough, leaving a 3-inch border.
Combine red currant jelly and 2 tablespoons water in a small microwave-safe dish, and cook on high for 30 seconds or until jelly melts.
Brush melted jelly mixture over plums. Fold 3-inch border of dough over plums, pressing gently to seal (dough will partially cover plums).
Combine 1 egg and 1 teaspoon water.
Brush dough with egg mixture, and sprinkle with turbinado sugar.
Bake at 400 for 25 minutes or until lightly browned. Cool tart on pan for 5 minutes. Carefully slide tart onto a platter using a spatula.
