Heat oven to 425°F. Line a large rimmed baking sheet with parchment paper.
Lay out puff pastry sheet on a lightly floured surface. With a rolling pin, gently roll out the pastry to a sheet 12x12-inches. Cut into 9 (4-inch) squares. Transfer the pastry squares to the parchment lined baking sheet.
Spoon on about 1 tablespoon of preserves on each square (not touching the pastry edges).
Start to arrange 2-3 brie slices (depends on how big your slices are) on top of the preserves.
Layer on 2-3 plum (or pluot) slices on top of the brie slices.
Make the egg wash by beating the egg and water in a small bowl. Brush egg wash over the border of each pastry. Sprinkle a little sugar evenly over the border.
Bake for 25-30 minutes or until the edges have puffed and are golden brown and crisp. The brie will melt and may spread over the pastry.
While the tarts are baking, combine the honey, lemon juice, and fresh thyme.
Let tarts rest a few minutes on the baking sheet.
When ready to serve, drizzle on the honey-thyme mixture.
Best served warm. Enjoy!