Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Slice the chicken breast into bite-sized strips and halve the cremini mushrooms, keeping them roughly the same size for even cooking.
In a small bowl, melt the ghee and whisk it together with the extra virgin olive oil, minced garlic, sea salt, black pepper, and fresh thyme.
Place the chicken and mushrooms on the prepared baking sheet and pour the garlic butter mixture over them, tossing thoroughly to ensure every piece is well-coated.
Spread the ingredients into a single layer, ensuring they aren't crowded so they roast rather than steam.
Roast in the center of the oven for 18-20 minutes, or until the chicken is cooked through and the mushrooms have developed a deep golden-brown color.
Remove the pan from the oven and immediately toss in the baby spinach, allowing the residual heat to wilt the leaves gently.
Drizzle with fresh lemon juice just before serving to brighten the savory flavors.