Preheat your oven to 350°F.
In a large mixing bowl, mix together the shredded chicken, sour cream, cream of chicken soup, frozen mixed veggies, parmesan cheese, pepper, and onion powder. Mix until thoroughly combined.
Transfer the poppy seed casserole mixture to a greased 9x13-inch casserole dish and smooth out the top.
Crush two sleeves of Ritz crackers over the casserole in an even layer, then pour the melted butter evenly over top of the cracker topping.
Bake the casserole for 30-35 minutes, or until it is hot and bubbly.
Allow the casserole to cool for a few minutes before serving. Serve as is or over a bed of rice alongside a garden salad.