Sprinkle the eggplant generously with salt and let it drain in a colander for 30 minutes. Rinse it well and pat it dry on towels.
Preheat your oven to 350°F.
Heat a large frying pan over medium-high heat. Once hot, add the oil and once the oil is shimmering, add the eggplant. Fry for 8 minutes, stirring occasionally. Transfer them to paper towels to drain. Add celery and carrots to remaining oil and cook for 3 minutes before adding onion and garlic. Cook together for 5 more minutes on medium heat.
Stir in the orzo and tomato paste and cook for two minutes more. Off the heat, add the oregano, mozzarella, parmesan, tomatoes, fried eggplant, lemon zest, salt, black pepper, and the stock. Mix well.
Transfer mixture to an ovenproof baking dish. Cover with foil and bake for 20 minutes, then bake for 20 minutes without the foil. Let rest for 5 minutes before serving.