Preheat the oven: Set your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with nonstick spray or butter.
Mix the filling: In a large mixing bowl, stir together the cream of chicken soup, sour cream, milk, garlic powder, onion powder, paprika, salt, and pepper. Add the cooked chicken and vegetables. Stir until everything is evenly coated.
Add the biscuits: Open the can of refrigerated biscuits, cut each into quarters, and gently fold them into the creamy mixture. Coat every piece — this helps them bake up soft inside but golden on top.
Assemble the casserole: Pour the mixture into your prepared baking dish. Spread it out evenly and sprinkle cheese on top.
Bake to bubbly perfection: Cover with foil and bake for 20 minutes. Then uncover and bake another 10–15 minutes, until the biscuits are puffed, golden, and cooked through. If the top browns too fast, loosely tent with foil.
Finish and serve: Brush the top with melted butter if desired. Let it cool for 5 minutes before serving — it thickens slightly as it rests.