Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Pat the chicken breast dry with a paper towel and season both sides evenly with the olive oil, half of the sea salt, and half of the black pepper.
Clean the cremini mushrooms with a damp cloth to remove any grit, then halve them so they are uniform in size for even roasting.
In a small bowl, whisk together the melted ghee, minced garlic, and fresh thyme leaves until well combined.
Place the chicken breast on one side of the prepared baking sheet and spread the mushrooms out on the other side in a single layer.
Pour the garlic ghee mixture over the mushrooms and toss them gently with a spatula to ensure every mushroom is well coated.
Roast in the oven for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are golden brown.
Remove from the oven, sprinkle the remaining salt and pepper over the mushrooms, and finish with a bright squeeze of fresh lemon juice before serving.