Preheat oven to 400 degrees. Line 2 baking sheets with parchment paper.

Dust counter or board with flour. Lightly roll or flatten one sheet of puff pastry and cut 9 squares with a sharp knife. (Keep other sheet of puff pastry in fridge while you work.)

With the tip of your knife, draw an outer boarder about 1/2 inch all along the outside of each pastry square. The plums should not go outside of this line.  Place squares on one of the baking sheets a few inches apart. With a fork, gently prick the center of each square a few times (called docking) to make sure your puff pastry, doesn't, well, puff up too much in the middle while baking!

Arrange plum slices and sprinkle each square with1/2 tbsp. of sugar. Repeat with other sheet of pastry dough, remaining plums, and sugar on the other other baking sheet.

With a fork, beat egg and 2 tbsp. water in a small bowl and brush over outer edges of tarts. Bake for 15-18 minutes or until pastry dough is golden brown. Check tarts around minute 15 so it does not burn. 

While tarts are baking, combine preserves and water in a small saucepan and stir until combined and the preserves are slightly thinned out. 

Remove from oven and let cool for 5 minutes and then gently brush plums with preserves/water mixture. Dust with Confectioner's sugar if desired and serve.