Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Trim the ends of the Brussels sprouts and slice them in half; dice the sweet potato into uniform 1/2-inch cubes.
Cut the chicken breast into 1-inch bite-sized pieces to ensure they cook at the same rate as the vegetables.
In a large mixing bowl, whisk together the extra virgin olive oil, minced garlic, dried oregano, sea salt, black pepper, and the zest and juice of the half lemon.
Add the chicken, Brussels sprouts, and sweet potatoes to the bowl, tossing thoroughly until every piece is coated in the herb marinade.
Spread the mixture in a single layer across the prepared sheet pan, ensuring the ingredients are not crowded so they roast rather than steam.
Roast for 20-25 minutes, tossing the ingredients halfway through, until the chicken is cooked through and the Brussels sprouts have developed golden, crispy edges.
Remove from the oven and serve immediately, finishing with an optional extra squeeze of fresh lemon for brightness.