Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Slice the chicken breast into 1-inch bite-sized pieces and halve the cremini mushrooms, ensuring they are all similar in size for even cooking.
In a small microwave-safe bowl, melt the ghee and whisk in the minced garlic, chopped fresh thyme, sea salt, and black pepper.
Place the chicken and mushrooms on the prepared baking sheet and pour the garlic butter mixture over them, tossing thoroughly to ensure every piece is well-coated.
Spread the mixture into a single layer, making sure not to overcrowd the pan so the mushrooms roast rather than steam.
Roast in the oven for 18-20 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are tender and golden brown.
Remove from the oven and immediately toss in the baby spinach, allowing the residual heat to wilt the leaves slightly.
Drizzle with fresh lemon juice before serving to brighten the savory flavors.