Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish.
Dice the chicken breast into bite-sized pieces and season with half of the sea salt, black pepper, and garlic powder.
Heat the olive oil in a skillet over medium-high heat. Add the chicken and sliced mushrooms, sautéing until the chicken is cooked through and mushrooms are browned.
Stir in the fresh baby spinach and cook for 1 minute until just wilted.
In the prepared baking dish, combine the cooked basmati rice, the sautéed chicken and mushroom mixture, chicken broth, and the remaining seasonings.
Stir well to ensure the rice is moistened by the broth, then flatten the top with a spoon.
Sprinkle the shredded mozzarella and grated parmesan evenly over the top.
Bake for 15 to 20 minutes, or until the cheese is melted, bubbly, and slightly golden on the edges.
Remove from the oven and let it rest for 2 minutes before serving.