Heat the oven to 450 degrees F/230 degrees C.
Heat 2 tablespoons of the olive oil in a saute pan and cook the onions until soft, about 5 minutes. Add the garlic, bay leaf, and pepperoncino, and cook 1 minute. Then add the tomatoes and cook to a thick, chunky sauce, about 30 minutes. Remove the bay leaf, season with salt and pepper and stir in the fresh herbs.
While the sauce cooks, brush the eggplant slices on both sides with olive oil and lay in a single layer on a baking sheet (you'll need to work in batches). Bake until golden and soft, about 15 minutes; turn and bake the other side, about 10 minutes. Remove and set aside.
Turn the oven down to 375 degrees F/190 degrees C. Mix together the breadcrumbs with the chili powder.
In a large gratin dish, starting with just the thinnest smear of tomato sauce over the bottom, layer in the eggplant, then sauce, then breadcrumbs. Repeat until the dish is full. Scatter the Parmesan over top. Bake until the gratin is heated through, and the top golden, about 30 minutes. Serve.