How to Make It
Preheat the oven to 425° and line a baking sheet with parchment paper. In a large skillet, heat the olive oil. Add the squash and onion and season with salt and white pepper. Cover and cook over moderately high heat, stirring occasionally, until the squash and onion are lightly browned, about 5 minutes. Remove the skillet from the heat and let stand, covered, for 5 minutes. Transfer the vegetables to a strainer and press lightly.
Meanwhile, in a small bowl, blend the pesto with the goat cheese. On a lightly floured surface, roll out the puff pastry to a 13-inch square; trim the square to 12 inches. Prick the pastry all over with a fork and invert it onto the parchment-lined baking sheet.
Spread the goat cheese all over the pastry, leaving a 1-inch border all around. Top with the squash mixture. Arrange the tomato slices on the tart and sprinkle with salt and white pepper. Fold up the sides, pressing the corners together. Trim any excess pastry at the corners. Brush the pastry with the egg wash and bake in the lower third of the oven for about 45 minutes, until the edges are golden and the bottom is completely cooked through. Sprinkle with the olives, cut into squares and serve right away.