Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Halve the baby potatoes and trim the woody ends off the asparagus spears.
In a small bowl, whisk together the olive oil, juice from the half lemon, minced garlic, dried oregano, sea salt, and black pepper.
Place the chicken breast and halved potatoes on the sheet pan and drizzle with two-thirds of the lemon-herb marinade, tossing to coat evenly.
Roast the chicken and potatoes for 15 minutes.
Remove the pan from the oven, add the asparagus spears to the empty space, and drizzle with the remaining marinade.
Return the pan to the oven and roast for an additional 10 to 12 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender.
Let the chicken rest for 5 minutes before slicing and serving alongside the roasted vegetables.