Preheat oven to 425°F.
Toss the eggplant, bell peppers, onion, and garlic with the olive oil, salt, and pepper on a large baking sheet. Roast for 40 minutes, or until browned, turning once with a spatula.
Meanwhile, cook orzo in boiling salted water for 7 to 9 minutes (according to package directions), until tender. Drain and transfer to a large serving bowl.
Add roasted vegetables to the orzo, scraping all the liquid and seasoning from the roasting pan into the pasta bowl.
For the dressing, combine lemon juice, olive oil, salt, and pepper, and pour on orzo/vegetable mixture. Cool to room temperature.
Add the scallions and basil, if using. Check the seasonings and adjust with salt and pepper if necessary (not necessary for my taste). Serve at room temperature.