Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper.
Clean the cremini mushrooms and cut them into quarters, then slice the chicken breast into one-inch bite-sized pieces.
In a small bowl, whisk together the melted ghee, minced garlic, and fresh thyme leaves until well combined.
Place the mushrooms and chicken on the prepared baking sheet, drizzle with the garlic-ghee mixture, and season with sea salt and black pepper.
Roast for 18 to 20 minutes, tossing the ingredients halfway through, until the chicken is cooked and the mushrooms are golden and tender.
Remove from the oven, immediately toss with the baby spinach and grated parmesan cheese until the greens wilt, then finish with a bright squeeze of fresh lemon juice.