1-First, heat about 1/8 inch of olive oil in a large skillet over medium heat. Slice the eggplant into 1/2-inch thick slices and arrange them in a single layer without overlapping. Season with salt and pepper, then cook until tender and browned on both sides, turning as needed, and drain on paper towels. Add more oil if necessary to keep the slices frying evenly.

2-Next, in the same skillet, cook the chopped onion in the remaining oil until tender. Add the chopped tomatoes, salt, and pepper, then simmer the sauce for 10 to 15 minutes until it thickens. Preheat your oven to 400°F (200°C) while the sauce cooks. This step builds a flavorful base for layering.

3-Now, assemble the dish in a gratin or baking dish by layering eggplant slices, tomato sauce, and grated Parmesan cheese, repeating 2 to 3 times. The total preparation and cooking time is about one hour or slightly more, so plan accordingly. Bake for about 15 minutes, or until the cheese is melted and bubbly.

4-Once baked, garnish with chopped parsley if desired, and serve hot or at room temperature. To incorporate variations, like using a Panini grill for less oil, try it as a simpler method. For a cleaner alternative, brush sliced eggplant with oil and bake it directly, which reduces mess while keeping the rich layered eggplant parmesan gratin intact.