Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Slice the chicken breast into bite-sized cubes and halve the cremini mushrooms, ensuring they are clean and dry.
In a small bowl, melt the ghee and whisk in the minced garlic, fresh thyme, sea salt, and black pepper.
Place the chicken and mushrooms on the prepared sheet pan and drizzle with the garlic-ghee mixture, tossing thoroughly to coat every piece.
Spread the mixture into a single layer to ensure even roasting and prevent steaming.
Roast in the oven for 18-20 minutes, or until the chicken is cooked through and the mushrooms are golden brown and tender.
Remove the pan from the oven and immediately toss in the fresh baby spinach, allowing the residual heat to wilt the leaves slightly.
Drizzle with fresh lemon juice before serving to brighten the flavors and cut through the richness of the ghee.