Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Rinse the dry quinoa under cold water, then combine it with 0.4 cups of water in a small saucepan; bring to a boil, then reduce to a simmer, cover, and cook for 12 minutes until fluffy.
Cut the chicken breast into bite-sized pieces and place them on the baking sheet along with the broccoli florets, sliced red bell pepper, and drained chickpeas.
Drizzle the olive oil over the chicken and vegetables, then sprinkle with sea salt, black pepper, and garlic powder, tossing well to ensure everything is evenly coated.
Roast the chicken and vegetable mixture in the oven for 18 to 20 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
While the tray roasts, whisk together the tahini and fresh lemon juice in a small bowl, adding a teaspoon of warm water at a time until the dressing reaches a smooth, pourable consistency.
Divide the cooked quinoa into a bowl, top with the roasted chicken and vegetable medley, and finish by drizzling the creamy lemon-tahini dressing over the top.