Preheat oven to 400F. Roll out puff pasty on a baking sheet lined with parchment and trim edges to form a neat rectangle. With a sharp knife, score the pastry about 1/2 an inch from the edge on all sides. With a fork, prick the middle section of the pastry multiple times.
Thinly slice the tomatoes and arrange in the middle section of the pastry. Crumble the feta and scatter over the tomatoes. Remove thyme leaves from their stems and scatter over the tomatoes. Drizzle with balsamic vinegar and season with salt and pepper.
Bake in the oven for 25 minutes or until pasty is golden brown and crispy. Remove from oven and place on cooling rack before cutting into 8 squares.