Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into uniform 1-inch cubes and halve the cremini mushrooms, leaving smaller ones whole.
In a small bowl, whisk together the melted ghee, minced garlic, fresh thyme, sea salt, and black pepper until well combined.
Arrange the chicken, mushrooms, and trimmed green beans on the prepared baking sheet in a single layer.
Drizzle the garlic ghee mixture over the chicken and vegetables, tossing thoroughly with your hands or tongs to coat every piece.
Roast in the center of the oven for 18 to 20 minutes, or until the chicken is cooked through and the mushrooms are beautifully caramelized.
Remove from the oven and finish with a squeeze of fresh lemon juice before serving immediately while hot.