In a large serving bowl, toss the onion, lemon juice, 2 teaspoons sumac and ¼ teaspoon salt; let stand. Meanwhile, in a 10-inch skillet over medium, toast the pita, stirring, until beginning to brown, 2 to 3 minutes. Drizzle with 1 tablespoon of the oil, then toss. Toast, stirring often, until the pieces are golden brown and crisp, about 5 minutes. Off heat, sprinkle with 1 teaspoon of the remaining sumac, then toss. Transfer to one side of a large paper towel-lined plate. Wipe out the skillet.

In a medium bowl, toss the chickpeas with the cornstarch. Transfer to a fine-mesh strainer and shake to remove excess. In the same skillet over medium-high, heat 4 tablespoons of the remaining oil until shimmering. Add the chickpeas and cook, stirring, until golden brown and crisp, 5 to 7 minutes. Using a slotted spoon, transfer to the other side of the paper towel-lined plate. Sprinkle the chickpeas with the remaining 2 teaspoons sumac and ¼ teaspoon each salt and pepper.

To the onion mixture in the serving bowl, stir in the remaining 2 tablespoons oil, tahini and honey. Add the tomatoes, cucumber, arugula and mint, then toss. Taste and season with salt and pepper. Add the pita and toss. Scatter the chickpeas over the salad, along with any sumac, salt and pepper remaining on the plate.