Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Scrub the russet potato clean and cut it into uniform half-inch cubes to ensure even roasting.
Mince the garlic cloves and finely chop the fresh rosemary needles.
In a medium mixing bowl, toss the potato cubes with half of the olive oil, half of the garlic, half of the rosemary, and the sea salt and black pepper.
Spread the seasoned potatoes in a single layer on one side of the prepared baking sheet.
Coat the chicken breast with the remaining olive oil, garlic, rosemary, and dried oregano, then place it on the other side of the baking sheet.
Roast in the center of the oven for 25 to 30 minutes, flipping the potatoes halfway through, until the chicken is cooked through and the potatoes are golden brown.