Preheat the oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Season the cubed chicken with salt, pepper, and onion powder.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Sear the chicken until browned on all sides (approximately 6-8 minutes). Transfer chicken to the prepared baking dish.
In the same skillet, add the remaining tablespoon of olive oil and the sliced mushrooms. Sauté for 8-10 minutes until the mushrooms have released their moisture and turned golden brown.
Add the minced garlic and dried thyme to the mushrooms and cook for 1 minute until fragrant.
Reduce heat to medium. Stir in the chicken bone broth, heavy cream, and cubed cream cheese. Whisk continuously until the cream cheese is fully melted and the sauce is thickened and smooth.
Add the fresh baby spinach to the skillet and stir until just wilted (about 1-2 minutes).
Pour the mushroom and cream sauce over the chicken in the baking dish. Toss gently to ensure even distribution.
Top the casserole evenly with shredded mozzarella and grated parmesan cheese.
Bake for 20-25 minutes until the sauce is bubbling and the cheese topping is golden brown. Let rest for 5 minutes before serving.