In a medium nonstick skillet, heat 2 teaspoons oil over medium-high heat. Add the eggplant and cook until tender, about 2 minutes. Transfer to a bowl. Repeat the procedure with the zucchini, using an additional 2 teaspoons oil, and cooking for 2 minutes. Repeat with the fennel, using another 2 teaspoons oil, and cooking for 2 minutes. Cool the vegetables completely. Add the tomatoes, olives, remaining 4 tablespoons oil, the vinegar, thyme, garlic, salt, and pepper and mix well. Cover and refrigerate until ready to serve, up to 8 hours.
For each serving, brush the inside of a large dinner plate with some of the oil. Place 1 tuna slice on the plate and cover with a large piece of strong plastic wrap. Using the flat side of a meat pounder, pound the tuna until it is so thin it covers the inside of the plate. Leaving the plastic wrap intact, refrigerate until ready to serve, up to 8 hours. When ready to serve, carefully peel off the plastic wrap. Brush the tuna with the remaining olive oil and sprinkle with the salt. Spoon the vinaigrette over the tuna, spreading it evenly over the surface. Serve immediately.