Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Chop the potato, carrots, onion, and celery into uniform 1-inch pieces to ensure even roasting.
Cut the chicken breast into 1-inch cubes, keeping them similar in size to the vegetables.
In a large mixing bowl, whisk together the olive oil, sea salt, black pepper, dried rosemary, and garlic powder.
Add the chopped chicken and vegetables to the bowl and toss thoroughly until every piece is coated in the herb oil.
Spread the mixture onto the prepared baking sheet in a single layer, ensuring they are not overcrowded so they roast rather than steam.
Roast for 25 to 30 minutes, tossing the medley halfway through, until the chicken is cooked through and the vegetables are golden-brown and tender.