Preheat your oven to a warm 400°F (200°C). Lightly grease a 9x13 inch casserole dish. Spread the cooked shredded chicken evenly across the bottom.
In a large skillet, melt the butter with the olive oil over medium heat until it shimmers. Add the sliced onion and chopped garlic, cooking until they're soft and fragrant, about 3-5 minutes.
Stir in the sliced white mushrooms and continue to cook until they're tender. Don't worry about any liquid from the mushrooms; it adds great flavor to your sauce!
Spoon this delicious mushroom, onion, and garlic mixture over the chicken in your casserole dish.
Return to the skillet and pour in the beef broth. Add the cubed cream cheese, stirring until it starts to melt into the broth.
Stir in the heavy cream, salt, and pepper. Continue stirring until the sauce is beautifully smooth. For an extra silky texture, you can use an immersion blender directly in the skillet.
Generously pour this creamy sauce over the chicken and mushrooms in your casserole dish. Finish by sprinkling the shredded mozzarella cheese evenly across the top.
Bake for 20-25 minutes, or until the casserole is hot, bubbly, and the cheese is golden and melted.
Serve & Enjoy!