1. Follow package direction and cook vermicelli.
2. Grease an 8-inch by 12-inch baking dish with cooking spray and set it aside.
3. Preheat oven to 400 degree F.
4. In a large bowl, combine cream of chicken soup, milk, Parmesan cheese, and onion powder. Mix well.
5. Drop chicken, mushrooms, and cooked vermicelli in the bowl. Mix to combine.
6. Into the baking dish transfer the mixture.
7. In a small bowl, combine dry breadcrumbs and melted butter.
8. Spread breadcrumb mixture evenly over the top of the chicken mixture in baking dish.
9. Place baking dish in oven and bake for 25 minutes or until the breadcrumbs begin to brown and the mixture is hot and bubbling.
10. Remove tetrazzini from the oven and serve immediately.
You can use leftover chicken for the dish. But if you do not have leftover then place few salted chicken breast in Crockpot and cook on high setting for 3 ½ hours. Let it cool and then cut into chunks for the dish. You can also freeze leftover in freezer for preparing casserole later.
Add 1 teaspoon of salt if desired while cooking vermicelli.