Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Clean the cremini mushrooms with a damp cloth to remove any debris and slice them into thick halves or quarters.
Place the chicken breast on one side of the baking sheet, drizzle with olive oil, and season with half of the sea salt and black pepper.
In a small bowl, whisk together the melted grass-fed butter, minced garlic, and fresh thyme.
Toss the mushrooms in the garlic butter mixture until evenly coated, then spread them in a single layer on the other side of the baking sheet.
Roast for 18-22 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are tender and caramelized.
Remove from the oven and season the roasted mushrooms with the remaining salt and pepper.
Serve the roasted chicken and golden mushrooms immediately over a fresh bed of baby spinach, allowing the residual heat to slightly wilt the greens.