Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Wash the Yukon Gold potato and dice it into uniform half-inch cubes, then finely mince the garlic clove and the fresh rosemary leaves.
In a mixing bowl, toss the potato cubes with half of the olive oil, half of the minced garlic, half of the rosemary, and a pinch of the sea salt and black pepper.
Spread the potatoes in a single layer on the prepared baking sheet and roast for 15 minutes to give them a head start on crisping.
While the potatoes roast, rub the chicken breast thoroughly with the remaining olive oil, garlic, rosemary, lemon zest, and the rest of the salt and pepper.
Carefully remove the baking sheet from the oven, slide the potatoes to one side, and place the seasoned chicken breast on the other side.
Return the sheet to the oven and roast for an additional 18-20 minutes, or until the chicken reaches an internal temperature of 165°F and the potatoes are golden and tender.
Remove from the oven and immediately drizzle the fresh lemon juice over both the chicken and the roasted potatoes.
Place the fresh baby spinach on a plate and top with the hot chicken and potatoes, allowing the residual heat to gently wilt the greens before serving.