Sprinkle eggplant rounds with 2 tsp salt flakes and set aside for 10 minutes to soften.
Meanwhile, combine mint, parsley, walnuts, 1/3 cup oil, and lemon juice in a bowl. Set aside.
Heat a barbecue grill or chargrill pan on medium heat. Use remaining oil to brush both sides of eggplant. Chargrill for 3-4 minutes on both sides or until charred and tender. Arrange eggplant on a platter. Top with tomato, feta, and extra herbs, and serve drizzled with dressing.