Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Clean the cremini mushrooms with a damp cloth and slice them into halves or quarters depending on size.
Cut the chicken breast into bite-sized 1-inch cubes to ensure even cooking with the vegetables.
In a large mixing bowl, whisk together the melted ghee, minced garlic, sea salt, black pepper, chopped thyme, and chopped rosemary.
Add the chicken cubes and mushrooms to the bowl, tossing thoroughly until every piece is well-coated in the herb ghee.
Spread the mixture in a single layer on the prepared baking sheet, ensuring no overcrowding so the mushrooms roast rather than steam.
Roast for 18 to 22 minutes, tossing halfway through, until the chicken is cooked through and the mushrooms have developed a deep golden-brown color.
Place the fresh baby spinach in a serving bowl and top immediately with the hot roasted chicken and mushrooms, allowing the residual heat to gently wilt the greens.