Bring a medium saucepan of salted water to a boil. Add the green beans and cook until crisp-tender, 2 to 3 minutes. Remove with tongs and transfer to a large bowl of ice water to stop the cooking; reserve the pan of water. When the green beans are completely cooled, remove and pat dry with paper towels.
Return the water to a gentle simmer. Carefully add the eggs and cook 10 minutes, then drain and rinse under cold water. Peel the hard-boiled eggs and cut into wedges.
Meanwhile, whisk the harissa, 1 tablespoon water, the lemon zest, lemon juice, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. Slowly whisk in the olive oil until smooth. Stir in the chickpeas and tomatoes and let marinate 10 minutes; season with salt.
Divide the romaine among bowls. Top with the marinated chickpea mixture, green beans, tuna, eggs and olives. Drizzle with any remaining dressing from the bowl. Serve with the flatbread.