Heat the oven to 425°F.
Toss 1 pound broccoli florets; ½ thinly sliced medium red onion; 1 (about 15-ounce) can drained, rinsed, and dried chickpeas; 3 tablespoons olive oil; 1 teaspoon kosher salt; 1 teaspoon ground cumin; ¾ teaspoon garlic powder; and ¼ teaspoon black pepper together on a rimmed baking sheet until well-coated. Spread into an even layer.
Roast until the broccoli is tender and browned, and the chickpeas look dry, 20 to 25 minutes. Meanwhile, make the dressing.
Whisk 3 tablespoons tahini, 2 tablespoons lemon juice, 2 tablespoons water, 1 small finely grated garlic clove, ¼ teaspoon kosher salt, and a few grinds black pepper together in a small bowl until smooth.
When the vegetables are ready, drizzle about half the dressing over them (still on the sheet pan) and toss to coat. Transfer to a serving bowl. Sprinkle 2 ounces crumbled feta cheese over the top and drizzle with the remaining dressing. Serve warm or at room temperature.
