Preheat your oven to 400 degrees Fahrenheit and line a large rimmed baking sheet with parchment paper.
In a large mixing bowl, combine the cubed chicken breast, halved cremini mushrooms, minced garlic, sea salt, black pepper, chopped thyme, and chopped rosemary.
Melt the ghee and pour it over the chicken and mushroom mixture, tossing thoroughly to ensure every piece is evenly coated in the herb fat.
Spread the mixture across the prepared baking sheet in a single layer, ensuring there is enough space between pieces to allow for proper roasting rather than steaming.
Roast for 18 to 20 minutes, or until the chicken reaches an internal temperature of 165 degrees Fahrenheit and the mushrooms are tender and golden.
Remove from the oven, drizzle with fresh lemon juice for brightness, and serve the warm mixture immediately over a fresh bed of baby spinach.