Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, combine the sliced mushrooms and broccoli florets. Sauté for about 5 minutes until slightly tender.
In a large mixing bowl, combine the shredded chicken, cooked rice, sautéed vegetables, cream of chicken soup, milk, garlic powder, onion powder, black pepper, and salt. Mix well until all ingredients are evenly combined.
Transfer the mixture to a 9x13-inch baking dish, spreading it out evenly.
If using, sprinkle the breadcrumbs on top of the casserole for added crunch.
Top the casserole with shredded cheddar cheese.
Bake in the preheated oven for 30-35 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for a few minutes before serving.