Preheat oven to 400°F and line a baking sheet with parchment paper.
Clean the mushrooms with a damp cloth and slice them in half; mince the garlic and finely chop the fresh herbs.
Cut the chicken breast into bite-sized cubes and place them in a large mixing bowl with the mushrooms.
Melt the ghee and whisk in the minced garlic, thyme, rosemary, sea salt, and black pepper.
Pour the garlic-herb ghee over the chicken and mushrooms, tossing thoroughly to ensure every piece is well-coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring no overcrowding to allow for proper browning.
Roast for 18-20 minutes, tossing halfway through, until the chicken is cooked through and mushrooms are tender and golden.
Remove from the oven and immediately toss with the fresh baby spinach and lemon juice until the greens are just slightly wilted.