Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Clean the cremini mushrooms with a damp cloth and slice them into thick halves or quarters.
Mince the garlic cloves finely and chop the fresh thyme and parsley.
In a small bowl, melt the ghee and whisk in the minced garlic and half of the sea salt.
Place the chicken breast on one side of the baking sheet and season both sides with the remaining salt, pepper, and fresh thyme.
Arrange the mushrooms on the other side of the sheet and drizzle the garlic-ghee mixture over them, tossing to coat thoroughly.
Roast in the oven for 18 to 22 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are golden brown.
During the last 2 minutes of roasting, scatter the baby spinach over the mushrooms to allow it to wilt slightly from the heat.
Remove from the oven, garnish the entire dish with fresh parsley, and serve immediately while hot.