Preheat your oven to 350°F (175°C).
In a large pan, heat olive oil over medium heat and sauté sliced mushrooms until they are tender and slightly browned, about 5-7 minutes.
Add diced chicken breasts to the pan and season with garlic powder, salt, and pepper. Cook until the chicken is no longer pink, about 6-8 minutes.
In a large mixing bowl, combine the cooked rice or pasta, sautéed chicken and mushrooms, cream of mushroom soup, milk, and half of the shredded cheese.
Transfer the mixture to a greased baking dish and spread it out evenly.
Top the casserole with the remaining shredded cheese.
Cover with aluminum foil and bake for 25 minutes, then remove the foil and bake for an additional 10-15 minutes until bubbly and golden brown.
Once baked, let it cool for about 5 minutes before serving.