Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper.
Clean the mushrooms with a damp cloth and halve any large ones, then slice the chicken breast into uniform 1-inch cubes.
In a large mixing bowl, whisk together the melted ghee, minced garlic, chopped thyme, chopped rosemary, sea salt, and black pepper.
Add the chicken and mushrooms to the bowl and toss thoroughly until every piece is well-coated in the herb ghee.
Spread the mixture onto the prepared baking sheet in a single layer, ensuring enough space for the mushrooms to roast rather than steam.
Roast for 18 to 20 minutes, tossing halfway through, until the chicken is cooked through and the mushrooms are tender and caramelized.
Place the fresh baby spinach in a serving bowl and top with the hot chicken and mushroom mixture to gently wilt the greens.
Garnish with the shaved parmesan and serve immediately while warm.