Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Clean the cremini mushrooms with a damp cloth and slice them in half; cut the chicken breast into uniform 1-inch cubes.
In a small glass bowl, whisk together the melted ghee, minced garlic, sea salt, black pepper, and fresh thyme leaves.
Place the mushrooms, chicken cubes, and broccoli florets on the prepared baking sheet.
Drizzle the garlic-ghee mixture over the ingredients and toss thoroughly with your hands or tongs to ensure every piece is well-coated.
Spread the mixture out into a single layer, ensuring there is space between the pieces to allow for proper roasting rather than steaming.
Roast in the center of the oven for 18 to 20 minutes, or until the chicken reaches an internal temperature of 165°F and the mushrooms are deeply caramelized.
Remove from the oven and immediately drizzle with fresh lemon juice to brighten the earthy flavors before serving.