PREHEAT THE OVEN TO 375F. Warm a tablespoon of oil in a large pan and add the onion, garlic and half the herbes de Provence, and salt lightly. Stew gently for about 5 minutes, then remove the onions to a gratin dish and spread them evenly over the bottom. Season with pepper. Warm another tablespoon of oil in the same skillet, raise the heat to medium-high and add the zucchini, a few pinches of salt and the rest of the herbes de Provence. Saute, stirring occasionally, until they start to color, about 10 minutes. Distribute the zucchini over the onions; arrange them casually. Tuck in the slices of tomato here and there, along with the olives. Drizzle the remaining oil over the top. Cover with foil and bake for 25 minutes. Remove the foil and continue to bake another 10 minutes or so, or until the squash is tender and the juices are reduced. Allow to cool and serve warm or tepid.