Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper.
Pat the chicken breast dry with a paper towel and cut into even 1-inch bite-sized pieces.
Clean the cremini mushrooms with a damp cloth to remove debris and slice them into halves.
In a large mixing bowl, combine the chicken, mushrooms, minced garlic, melted ghee, sea salt, black pepper, fresh thyme, and chopped walnuts.
Toss the mixture thoroughly until every piece is well-coated in the garlic ghee and seasonings.
Spread the ingredients in a single layer on the prepared baking sheet, ensuring they are not overcrowded to allow for proper browning.
Roast for 18 to 20 minutes until the chicken is cooked through and the mushrooms are tender and caramelized.
Remove from the oven and immediately toss with the fresh lemon juice and baby spinach until the greens are just wilted from the residual heat.