Spread eggplant slices in one layer, sprinkle with salt (on both sides) and set aside for 45 minutes to allow the eggplants to sweat and lose their bitterness.
Preheat the broiler with the rack 4 inches from the element. Pat eggplant slices dry and arrange as many slices as possible in one single layer on a parchment-lined baking sheet. Brush slices generously with olive oil on both sides. Broil until the eggplant is golden on top, turn and broil until the second side is golden. Set the eggplant slices aside and repeat the process until all the eggplant is done. This will take a total of 8-10 minutes per batch.
Turn off the broiler, and pre-heat your oven to 400°F (200°C) moving the rack to the middle/upper third.
In a 4-quart saucepan, heat 2 tablespoons of olive oil and sauté onion until golden. Add chopped tomatoes, and sauté for 3 minutes. Add herbs, lavender buds, red pepper flakes, a splash of wine, salt, and black pepper, and cook for 2 minutes. Add the passata and cook over medium heat for 10-15 minutes, or until thick.
Lightly coat a gratin dish with olive oil. Arrange a third of the eggplant In the bottom of the dish. The slices should overlap slightly. Spoon a third of the tomato sauce over the eggplant followed by a third of the Parmigiano-Reggiano. Repeat twice, ending with the tomato sauce and Parmigiano-Reggiano. Top with breadcrumbs, a drizzle of oil, and salt and pepper to taste.
Bake for 35-40 minutes, until golden brown.