Prep Your Dish: Get your oven ready by preheating it to 400°F (200°C). Lightly grease a 9x13 inch baking dish and spread your cooked shredded chicken evenly at the bottom.
Sauté Aromatics: In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onion and minced garlic, cooking until they become soft and fragrant, about 3-5 minutes.
Cook Mushrooms: Stir in the sliced mushrooms and continue to cook until they've softened and released their liquid. Remove the mushroom mixture from the skillet and set aside; leave any remaining liquid in the pan.
Make Creamy Sauce: Add the beef broth and cubed cream cheese to the same skillet. Stir in the heavy cream, salt, and pepper. Cook over medium heat, stirring constantly, until the sauce is smooth and creamy. A quick blend with an immersion blender can help achieve extra smoothness if needed.
Assemble Casserole: Sprinkle the cooked onion, garlic, and mushroom mixture over the chicken in your baking dish.
Pour & Top: Pour the rich, creamy sauce evenly over the chicken and vegetables. Finish by generously sprinkling the shredded mozzarella cheese on top.
Bake to Perfection: Bake in the preheated oven for 20-25 minutes, or until the casserole is bubbly and the cheese is golden and melted.
Serve & Enjoy: Let it cool slightly, then scoop out generous portions and savor this comforting, healthy meal!