Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Pat the chicken breast dry with paper towels to ensure a good sear, then slice into uniform 1-inch bite-sized pieces.
Clean the mushrooms with a damp cloth to remove debris and slice them into thick halves or quarters depending on size.
In a large mixing bowl, whisk together the melted ghee, minced garlic cloves, sea salt, black pepper, and fresh thyme.
Add the chicken and mushrooms to the bowl, tossing thoroughly until every piece is evenly coated in the garlic butter mixture.
Spread the mixture in a single layer on the prepared baking sheet, ensuring there is enough space between pieces so the mushrooms roast rather than steam.
Roast for 18-20 minutes, or until the chicken is cooked through and the mushrooms are beautifully browned and tender.
Remove from the oven and immediately toss the hot mixture with the fresh baby spinach, allowing the residual heat to gently wilt the greens before serving.