Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the cremini mushrooms with a damp cloth to remove debris and slice them into even halves.
Mince the garlic cloves finely and strip the leaves from the fresh thyme sprigs.
In a small bowl, whisk together the melted ghee, minced garlic, thyme, and half of the sea salt and black pepper.
Place the mushrooms on the prepared baking sheet, drizzle with half of the garlic ghee mixture, and toss until every mushroom is well-coated.
Season the chicken breast evenly on both sides with the remaining sea salt and black pepper.
Heat a cast-iron skillet over medium-high heat and sear the chicken for 5 minutes per side until a deep golden crust forms.
Slide the mushrooms into the oven and roast for 18 minutes until they are tender and the garlic is aromatic and lightly browned.
Place the seared chicken in the oven on a separate tray for the final 8 minutes of the mushroom roasting time to ensure it reaches an internal temperature of 165°F.
Drizzle the roasted mushrooms with fresh lemon juice and serve immediately alongside the sliced pan-seared chicken.