Preheat your oven to 400°F (200°C).
Season the chicken breast evenly on both sides with half of the sea salt and black pepper.
Heat a large oven-safe cast-iron skillet over medium-high heat and melt half of the ghee.
Sear the chicken for 3 minutes per side until a golden crust forms, then remove the chicken from the pan and set aside.
In the same skillet, add the remaining ghee, mushrooms, minced garlic, and fresh thyme, tossing to coat the mushrooms thoroughly.
Place the chicken back into the skillet, nestled among the mushrooms, and transfer the pan to the oven.
Roast for 12 to 15 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are tender.
Remove the skillet from the oven, stir in the baby spinach and lemon juice, allowing the residual heat to wilt the greens before serving.