Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Pat the chicken breast dry with a paper towel and season both sides with half of the sea salt and black pepper.
In a mixing bowl, toss the sliced cremini mushrooms with olive oil, minced garlic, and the remaining salt and pepper until well coated.
Place the seasoned chicken breast and the garlic mushrooms in a single layer on the prepared baking sheet.
Melt the ghee in a small bowl and stir in the chopped fresh thyme, then drizzle half of this mixture over the chicken and mushrooms.
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 and tender.
Remove from the oven and immediately drizzle the balsamic vinegar and the remaining garlic-thyme ghee over the roasted components.
Arrange the fresh baby spinach on a plate and top with the hot chicken and mushrooms, allowing the residual heat to gently wilt the greens before serving.