Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper for easy cleanup.
Peel and dice the sweet potato into 1/2-inch cubes, then toss them on the pan with 0.5 tablespoon of the olive oil.
Roast the sweet potatoes for 10 minutes to give them a head start while you prep the remaining ingredients.
In a small bowl, whisk together the remaining 1 tablespoon of olive oil, lemon juice, lemon zest, minced garlic, dried oregano, sea salt, and black pepper.
Remove the pan from the oven and add the chicken breast, broccoli florets, and sliced red onion around the sweet potatoes.
Drizzle the lemon-herb mixture over the chicken and vegetables, tossing the veggies slightly to coat evenly.
Return the pan to the oven and roast for 18-22 minutes until the chicken reaches an internal temperature of 165°F and the vegetables are tender-crisp.