Preheat oven to 400F and line 2 baking sheets with parchment paper.
Cut the puff pastry sheet in half to form two medium sized rectangular tarts (or keep it as one large tart, if desired) and place each puff pastry sheet on a separate baking sheet.
Using a sharp knife, score the puff pastry to form a 1/2 inch border all the way around, careful not to cut all the way through the puff pastry. Using a fork, poke holes in the center portions of the puff pastry, but not on the borders.
Using an offset spatula or the back of a spoon, spread the Boursin cheese inside the border of the puff pastry.
Prepare the tomatoes by slicing the heirloom tomatoes into 1/4 inch slices and halving the grape tomatoes. Toss the tomatoes with the olive oil, balsamic vinegar and salt and pepper.
Arrange the tomatoes in the center of the puff pastry and sprinkle the tarts with the crumbled goat cheese.
In a small bowl, whisk the egg with 1 tablespoon of water to make an egg wash. Brush the egg wash on the border of the puff pastry tart.
Bake in the oven for 20-25 minutes, until the puff pastry is a nice golden brown. Sprinkle with fresh herbs, slice and serve.