Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper.
Dice the chicken breast into 1-inch cubes and halve the cremini mushrooms, ensuring they are uniform in size for even roasting.
In a small bowl, whisk together the ghee, minced garlic, thyme, rosemary, sea salt, and black pepper until well combined.
Place the chicken and mushrooms on the prepared baking sheet and drizzle with the garlic ghee mixture, tossing thoroughly to coat every piece.
Spread the mixture into a single layer, ensuring no pieces are overlapping to allow for proper browning.
Roast for 18 to 20 minutes, or until the chicken is cooked through and the mushrooms have developed a deep golden color.
Remove from the oven and immediately toss the hot chicken and mushrooms with the fresh baby spinach until the leaves are just wilted.
Drizzle the entire dish with fresh lemon juice and serve immediately while hot.