Preheat your oven to 400°F (200°C).
Pat the chicken breast dry and season both sides evenly with sea salt, black pepper, garlic powder, and dried oregano.
Heat the olive oil in a large oven-safe skillet over medium-high heat until shimmering.
Place the chicken in the skillet and sear for 3-4 minutes until a golden-brown crust forms, then flip.
Trim the woody ends off the asparagus and arrange the spears around the chicken in the skillet.
Squeeze the juice of half a lemon over the chicken and vegetables, then slice the remaining lemon and place the rounds on top.
Transfer the skillet to the oven and roast for 10-12 minutes, or until the chicken reaches an internal temperature of 165°F.
Remove the skillet from the oven, toss in the baby spinach, and stir gently for 30 seconds until the residual heat wilts the greens.
Let the chicken rest for 3 minutes before slicing and serving alongside the roasted vegetables.