Preheat the oven to 375°F (190°C).

Heat olive oil in a large skillet over medium heat. Sauté the sliced mushrooms for about 5-7 minutes until they are golden and tender.

In a large mixing bowl, combine the cream of mushroom soup, milk, garlic powder, salt, and pepper. Mix until smooth.

In a greased 9x13 inch casserole dish, layer the uncooked rice on the bottom. Scatter the sautéed mushrooms and chicken breasts over the rice.

Pour the soup mixture evenly over the chicken and rice, ensuring that everything is covered.

Sprinkle the shredded cheddar cheese over the top of the casserole.

Cover the casserole dish with aluminum foil and bake in the preheated oven for 45 minutes.

Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.

Let the casserole sit for 5-10 minutes before serving, allowing the flavors to meld and the casserole to set.