Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Pat the chicken breast dry with a paper towel and cut into uniform 1-inch cubes to ensure even cooking.
Clean the cremini mushrooms with a damp cloth and halve them, or leave them whole if they are small.
In a small bowl, whisk together the melted ghee, minced garlic, chopped thyme, rosemary, sea salt, and black pepper.
Place the chicken and mushrooms in a large mixing bowl and pour the garlic-herb ghee over them, tossing thoroughly to coat every piece.
Spread the mixture in a single layer on the prepared baking sheet, ensuring they are not crowded so they roast rather than steam.
Roast for 18 to 20 minutes, flipping halfway through, until the chicken is cooked through and the mushrooms are golden and tender.
Remove from the oven and drizzle with fresh lemon juice before serving to brighten the savory flavors.