For the sauce: Place the onion, garlic, hot pepper if using and oil in a pot over medium-high heat and cook until softened. Add the tomato sauce and bring to a simmer. Taste for seasoning and add salt and pepper as desired. Lower the heat and simmer for 30 minutes, while you prepare the eggplant.
For the eggplant: Cut the top and bottom ends off each eggplant, then cut a thin slice off one long side. Slice lengthwise into 1/2-inch thick slices, 5 to 6 per eggplant. You¿ll need three shallow bowls ¿ the one holding the flour, one the eggs and one the breadcrumbs. Sprinkle some salt and pepper into the eggs and whisk until blended. Dip both sides of each eggplant slice in the flour, then in the egg and then in the breadcrumbs to coat completely. Place on a tray until all slices are done. Add enough oil to cover the bottom of 1 or 2 large skillets. Add a garlic clove and place over high heat. When the garlic begins to sizzle, add 2 to 3 slices of eggplant per skillet, not overlapping. Fry both sides until a deep golden brown, 2 to 3 minutes per side, adjusting the heat as needed. Transfer to paper towels. Repeat until all the slices are done.
For the parmigiana: Preheat the oven to 400 degrees F. Pour some of the tomato sauce into a 10 by 10 by 2 1/2-inch glass or ceramic baking dish to thinly cover the bottom. Add 2 to 3 slices eggplant in a single layer. Top with more sauce, then a quarter each of the mozzarella and Parmesan. Repeat three times, ending with cheese. Cover loosely with foil and bake 30 minutes. Remove the foil and bake until bubbly and just beginning to brown, about 10 minutes more. Remove from the oven and let rest 10 minutes. Slice into portions and serve while hot.