To make the dough, in a food processor, combine the flour, cornmeal and ½ teaspoon salt. Add the butter and pulse until the mixture resembles coarse crumbs. Add the egg and ¼ cup (2 fl. oz./60 ml.) cold water and pulse just until the dough begins to come together. Flatten the dough into a disk, warp in plastic wrap, and refrigerate for at least 1 hour or up to 2 days.

In a large frying pan, warm the oil over medium-high heat. Add the zucchini and sprinkle lightly with salt and pepper. Reduce the heat to medium-low and cook, stirring frequently, until the zucchini is softened but not browned, about 5 minutes. Transfer the zucchini to paper towels to drain.

Position a rack in the lower third of the oven and preheat to 375°F (190°C). On a lightly floured work surface, roll out the dough into a 12-inch (30-cm.) circle about ¼ inch (6 mm.) thick. Carefully transfer the dough to a 9-inch (23-cm.) fluted tart pan with removable sides. Press the dough into the bottom and sides of the pan. Fold the edges over and press into the sides of the pan, forming a double thickness around the pan rim. Trim off any excess dough overhanging the rim.

Sprinkle half of the cheese over the bottom of the crust. Arrange the tomatoes and zucchini in overlapping concentric circles or another attractive pattern on top of the cheese. Season lightly with salt and pepper. Top with the remaining cheese and the thyme.

Bake until the crust is golden brown and the juices are bubbling, 35-40 minutes. Let the tart cool for 10 minutes. Remove the pan sides, sprinkle with thyme, and serve warm.