Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Clean the cremini mushrooms with a damp cloth and slice them into halves or quarters depending on their size.
Cut the chicken breast into uniform 1-inch cubes to ensure they cook evenly with the mushrooms.
In a small bowl, whisk together the melted ghee, olive oil, minced garlic, sea salt, black pepper, and fresh thyme leaves.
Place the chicken and mushrooms in a large mixing bowl and pour the garlic-ghee mixture over them, tossing thoroughly to coat every piece.
Spread the mixture onto the prepared baking sheet in a single layer, ensuring the pieces aren't overcrowded so they roast rather than steam.
Roast for 18 to 20 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are tender and golden.
Remove from the oven and immediately toss with the fresh baby spinach, allowing the residual heat to wilt the greens before serving.