If you didn't want to bake a big batch, you could easily half this recipe to serve 4. You can roast the eggplant ahead of time and store in the fridge until you are ready to make the orzo.
Preheat oven to 400 degrees and line two baking sheets with parchment paper. Spread eggplant cubes evenly over sheets and drizzle with 1 tbsp olive oil. Roast in the oven until tender, about 20 minutes. Set aside.
Heat remaining 2 tbsp olive oil in a large skillet over medium high heat. Add carrots, celery, onion, and garlic and cook for about 5 minutes, or until tender. Stir in the orzo, tomato paste, and roasted eggplant and cook for another two minutes.
Remove pan from heat and add half of the oregano, mozzarella, parmesan, lemon zest, water, and salt and pepper. Mix until combined.
Spray 2 8 x 8 baking dishes with cooking spray and divide orzo mixture evenly among them. Lay sliced tomatoes out evenly along the top and sprinkle with remaining oregano, and salt and pepper.
Reduce oven temperature to 350 degrees and cover baking dishes with foil. Bake for 20 minutes with foil on and then another 25 minutes with foil off. Let sit for about 5-10 minutes before serving so orzo can firm up.
Once cooled, you can cover tightly and freeze as is, or separate into individual servings and freeze.