1. Preheat oven to 425 degrees F.
2. Heat 2 tablespoons oil in a heavy-bottomed pot over medium-high heat. Cook scallion, cumin, coriander, and cayenne. Stir until fragrant (1 minute).
3. Add barley and stir until it smells nutty (2 minutes). Add broth, water, and 1/2 teaspoon salt. Bring to a boil. Reduce heat and simmer, covered, until all of liquid is absorbed and barley is tender (30-40 minutes).
4. While barley is cooking, cut eggplant and zucchini into bite-sized cubes. Toss with pepper, remaining 1/2 teaspoon salt, and 5 tablespoons oil in a bowl. Spread on two baking pans and roast, stirring occasionally, until vegetables are golden brown and tender (20-25 minutes).
5. Whisk together lemon juice, garlic, sugar, and remaining 3 tablespoons oil in a large bowl.
6. Add barley, roasted vegetables, and remaining ingredients to bowl with dressing. Toss until well combined and serve warm.