Combine the corn and chickpeas in a medium bowl. Add the coriander, 1 tablespoon of the oil, 1 teaspoon salt and a couple turns of pepper and mix well. Spread on a rimmed baking sheet and roast until lightly browned, about 30 minutes.
Whisk together the vinegar and Dijon mustard in a small bowl. Slowly drizzle in the remaining 4 tablespoons oil. Add the corn mixture, tomatoes and scallions and toss to combine.
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