Purée onion, herbs, 2 Tbsp oil, lemon juice and a bit of salt and pepper in a food processor. Chill until ready to use.
For the Tian:
Preheat grill to medium high heat. Grill eggplant and zucchini slices for just 3 minutes on either side, then remove to a large dish. Toss vegetables with Herb de Provence Rub, as well as with minced garlic.
Preheat oven to 375 °F (or turn grill to medium). Brush an 8 or 9-inch cast iron skillet with olive oil and arrange eggplant, zucchini and tomato slices in a circular fashion, overlapping each other closely, inserting basil leaves and olives occasionally between the slices. Season lightly and place a circle of parchment paper directly over the tian. Bake or grill roast the tian for about 40 minutes, until vegetables are tender, and most of the liquid from the tomatoes has been absorbed. Let tian sit for at least 15 minutes, before serving.