On a large rimmed baking sheet, combine the broccoli with 1 tablespoon of oil, the broth, half of the garlic, the cumin, 1/2 teaspoon of the salt, 1/2 teaspoon of the pepper, and the chili powder. Toss to coat, then spread the broccoli out in a single layer. Roast for 10 minutes.
Meanwhile in a large bowl, combine the chicken with remaining 1 tablespoon oil, remaining garlic, the lemon zest, remaining 1/2 teaspoon pepper. Toss to coat.
Remove the baking sheet from the oven, add the chicken and toss together with the broccoli. Return the sheet to the oven and roast, tossing once halfway through cooking time, until the chicken is cooked through and the broccoli is tender and golden around the edges, 8-10 minutes.
Serve with lemon wedges or squeeze the lemon juice over the chicken and broccoli just before serving.