Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch bite-sized pieces and halve any large cremini mushrooms so they cook evenly.
In a small bowl, whisk together the melted ghee, minced garlic, chopped thyme, and chopped rosemary until well combined.
Place the chicken and mushrooms on the prepared baking sheet, then drizzle the garlic-ghee mixture over the top.
Sprinkle with sea salt and black pepper, then toss thoroughly with your hands or a spatula to ensure every piece is well coated.
Spread the mixture into a single layer, ensuring the chicken pieces are not touching for maximum browning.
Roast for 18 to 20 minutes, or until the chicken is cooked through and the mushrooms are caramelized and tender.
In the final 2 minutes of roasting, add the baby spinach to the tray and toss gently to allow it to wilt in the residual heat.
Remove from the oven, garnish with freshly chopped parsley, and serve immediately.