Preheat oven to 350°F (175°C).
Prepare the chicken: If you haven’t already, cook and shred the chicken. You can use rotisserie chicken for a shortcut or bake/grill your own chicken breasts and shred them.
Mix the base: In a large bowl, combine the cream of chicken soup, sour cream, milk, shredded cheddar cheese, garlic powder, onion powder, salt, pepper, and thyme. Stir until smooth and well-combined.
Add the vegetables and pasta: Stir in the frozen mixed vegetables and cooked pasta. Make sure everything is well coated with the creamy mixture.
Combine with chicken: Add the shredded chicken to the mixture and stir to combine.
Assemble the casserole: Grease a 9x13-inch baking dish. Pour the chicken mixture into the dish and spread it evenly.
Prepare the topping: In a small bowl, mix the crushed Ritz crackers or breadcrumbs with the melted butter. Sprinkle this mixture evenly over the casserole.
Bake: Place the casserole in the preheated oven and bake for about 25–30 minutes, or until the casserole is bubbly and the topping is golden brown.
Serve: Let the casserole cool for a few minutes before serving. It pairs nicely with a side salad or steamed veggies.