Toss the broccoli florets, carrots, bell pepper and onion together with 2 tablespoons olive oil and some salt and pepper and spread out in an even layer on a sheet tray. Roast until the vegetables are softened slightly and browned, 15 to 18 minutes.
Meanwhile, bring the chicken stock to a boil in a small saucepot. Sprinkle the chickpeas and couscous in an even layer on top of the vegetables, drizzle with the remaining tablespoon oil and sprinkle with some salt and pepper. Pour the boiling stock over the top of the couscous and vegetables and cover with aluminum foil. Let sit until the couscous is cooked through, 5 to 8 minutes. Remove the foil and fluff the couscous with a fork. Sprinkle with the red pepper flakes, drizzle lemon juice over the top and garnish with the mint leaves.
To reheat the couscous before serving, cover with foil and place in a 350 degree F oven for 10 minutes.