How to Make It
Preheat the oven to 325°. Line a cookie sheet with parchment paper. On a lightly floured surface, roll out the puff pastry 1/16 inch thick. Cut out a 12-inch round and transfer it to the cookie sheet. Refrigerate for at least 20 minutes.
Arrange the tomatoes on a rimmed baking sheet, season with salt and pepper and bake for 1 hour.
Meanwhile, in a large skillet, heat 2 tablespoons of the olive oil. Add the leeks, season with salt and pepper and cook over low heat, stirring occasionally, until tender, about 10 minutes. Let cool to room temperature.
In a bowl, toss the potatoes with the remaining 1/2 tablespoon of olive oil and season with salt and pepper. When the tomatoes are done, slide them to 1 side of the baking sheet and scatter the potatoes on the other. Bake at 350° for 20 minutes, or until the potatoes are tender and golden and the tomatoes are lightly browned. Let cool.
Turn the oven up to 400°. Spread the leeks on the puff pastry round to within 1 inch of the edge. Top with the tomatoes, potatoes and thyme. Bake for about 30 minutes, or until the edge is puffed and golden. Scatter the olives over the galette, cut into wedges and serve.