Heat the oven: Preheat to 425°F (220°C). Place a large sheet pan on the middle rack while the oven heats.
A hot pan helps the potatoes crisp up.
Prep the vegetables: In a large bowl, toss sweet potato cubes and red onion wedges with 2 tablespoons olive oil, ½ teaspoon salt, and half the garlic. Add Brussels sprouts or green beans if using.
Season the chicken: Pat the thighs dry with paper towels. In a small bowl, mix smoked paprika, cumin, dried thyme, 1 teaspoon salt, black pepper, and the remaining garlic.
Rub spices all over the chicken. Drizzle with 1 tablespoon olive oil to help the seasoning stick.
Arrange on the pan: Carefully remove the hot pan and lightly oil it. Spread the vegetables in a single layer.
Nestle the chicken thighs on top, skin side up. This lets the chicken juices flavor the potatoes while the skin gets crispy.
Roast, then rotate: Roast for 20 minutes. Rotate the pan and stir the vegetables for even browning.
Roast another 15–20 minutes, until the chicken skin is crisp and an instant-read thermometer in the thickest part reads 165°F (74°C).
Finish with brightness: Zest the lemon over the pan. Squeeze half the lemon juice on top. If you like sweet heat, warm 1 tablespoon honey with a pinch of red pepper flakes and drizzle lightly over the potatoes.
Rest and serve: Let the chicken rest 5 minutes so the juices settle.
Sprinkle chopped parsley or cilantro. Taste and add a pinch of salt or another squeeze of lemon if needed.