Preheat oven to 400°F/200°C
Heat the olive oil in a large sauté pan over medium-high heat. Add the eggplant and cook for 8 minutes, stirring occasionally, until golden brown. Remove with a slotted spoon to paper towels and set aside
Add the carrots and celery to the pan and dry for 8 minutes. Turn down the heat to medium and add the onion and garlic. Cook for 5 minutes, stirring often
Add the orzo and tomato paste and cook for a further 2 minutes
Remove the pan from the heat and add the stock, fresh oregano, and lemon zest. Add the cooked vegetables, mozzarella, Parmesan, ½ teaspoon salt, and ½ teaspoon pepper. Mix well and transfer to a rectangular baking dish, 8½ by 10½-inches (21 by 27-cm) or a round one, 10.5 inches (27 cm) in diameter
Arrange the tomatoes on top and sprinkle with the dried oregano, ¼ teaspoon salt, and a grind of black pepper
Bake for 40 minutes, until all the liquid has been absorbed and the pasta is cooked through. Remove, leave to settle for 5 minutes, and serve