Homemade tomato sauce
Place a saucepan over medium-high heat. Add 2 tablespoons of olive oil, diced onion, and diced garlic. Cook for about 2 - 3 minutes. Add diced tomatoes and season with salt, pepper, and vinegar. Cook for another 10 - 15 minutes or until the sauce is thick and tomatoes are cooked.
Grill the eggplants
Place a grill pan over high heat (or use an outdoor grill). Cut the eggplants on about 2 cm or 1-inch thick steaks. Season with salt, pepper, and drizzle with 1 tablespoon of olive oil. Transfer the steaks to a smoking hot grill pan (you don't need to use any oil), Grill for about 2 - 4 minutes on each side, depending on the size.
Grilled Eggplant Steaks with Mozzarella Cheese and Tomato Sauce
Top each grilled eggplant steak with 1 or 2 tablespoons of your homemade tomato sauce and about one teaspoon of the Gremolata sauce. To finish it off, place a slice of Mozzarella cheese on each steak. Cover with a lid and cook until the Mozzarella cheese has melted, for approximately 3 - 4 minutes. Serve immediately. Optionally sprinkle with fresh chopped parsley, chopped walnuts or drizzle with Gremolata sauce. Enjoy.