Preheat and prep: Heat your oven to 425°F (220°C). Line a large sheet pan with parchment for easier cleanup. Pat the chicken dry with paper towels.
Season the sweet potatoes: In a large bowl, toss the sweet potato cubes and red onion with 2 tablespoons olive oil, 3/4 teaspoon salt, 1/4 teaspoon pepper, 1 teaspoon smoked paprika, and 1/2 teaspoon cumin.
Spread them in a single layer on the sheet pan.
Start roasting the veggies: Roast for 12–15 minutes to give the sweet potatoes a head start so they caramelize without overcooking the chicken.
Season the chicken: While the veggies roast, toss the chicken with 1 tablespoon olive oil, minced garlic, remaining smoked paprika and cumin, dried thyme, 3/4 teaspoon salt, and remaining pepper. Add lemon zest for extra brightness.
Add chicken to the pan: Pull the pan from the oven, scoot the sweet potatoes to make space, and nestle the chicken pieces among them. If using bell pepper, add it now.
Roast until done: Return to the oven for 15–20 minutes, until the chicken registers 165°F (74°C) in the thickest part and the sweet potatoes are tender and browned at the edges.
Finish with lemon: Squeeze half a lemon over everything.
Taste and adjust salt, pepper, or lemon as needed. If you like a slight glaze, drizzle a teaspoon of maple syrup and toss quickly.
Rest and serve: Let the chicken rest for 3–5 minutes. Garnish with chopped parsley or cilantro.
Serve as is, or with a side salad, quinoa, or steamed greens.