Cook the Grains: In a medium saucepan, combine farro and 4 cups water/broth. Bring to a boil, then reduce to low; cover and simmer 10–12 minutes. Stir in quinoa and 1 more cup water/broth. Continue simmering, covered, 12–15 minutes until liquid is absorbed and grains are tender; add hot water by ¼ cup if needed. Rest covered 5 minutes, then fluff. Doneness: farro tender with slight chew (no chalky core); quinoa translucent with visible tails.
Roast the Chickpeas: Preheat oven to 400°F (200°C). In a bowl, mix spices and oil; toss in well-dried chickpeas to coat. Spread on a rimmed sheet in one layer. Roast 25–30 minutes, shaking halfway, until golden and crisp.
Prepare the Vegetables: Chop cucumber, halve tomatoes, dice bell pepper, slice onion, and halve olives. Chop herbs.
Make the Tahini Drizzle: Whisk tahini, lemon juice, and garlic to a paste. Slowly whisk in cold water until smooth and pourable; whisk in olive oil and optional sumac.
Assemble: Divide the farro-quinoa blend among 6 bowls. Add crispy chickpeas and arrange cucumber, tomatoes, bell pepper, red onion, and olives.
Finish: Drizzle with tahini sauce, garnish with herbs and toasted sesame.