Preheat oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Pat the chicken breast dry and cut into uniform 1-inch cubes to ensure even cooking.
In a large bowl, combine the chicken cubes, halved cremini mushrooms, melted ghee, minced garlic, thyme, rosemary, salt, and pepper.
Toss thoroughly until every piece is well-coated in the herb-infused fat.
Spread the mixture in a single layer on the prepared baking sheet, ensuring no crowding to allow for proper browning.
Roast for 18 to 20 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are tender.
Remove from the oven and immediately toss with the fresh spinach and lemon juice, allowing the residual heat to wilt the greens.