Preheat oven to 400 degrees. Roll out thawed dough onto a floured surface to create a 9x9 square. Set the dough on a piece of parchment paper on a baking sheet. Crimp the edges of dough to make a decorative edge.
Set a piece of foul over the dough and fill with dried beans. Leave the edges exposed. In a small bowl whish together the egg and water. Use a pastry brush to lightly coat the edges with egg wash.
Bake the tart dough for 8 minutes. Remove the foil with the dried beans and use a fork to pierce the bottom of the tart. Return the tart to the oven and bake for 10 minutes or until cooked through and lightly browned. Cool to room temperature.
For the filling:
Heat the olive oil in a large sauté pan set over medium heat. Add the sliced onions and cook stirring often until soft and caramelized, about 15 to 18 minutes. Remove the onions from the pan.
To assemble the tart begin by spreading the mascarpone cheese over the shell. Top with a layer of caramelized onions then arrange the plum slices. Drizzle with honey, balsamic vinegar and garnish with thyme.