Preheat the oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or olive oil.
In a large bowl, combine the diced potatoes, chicken pieces, garlic powder, onion powder, paprika, salt, and pepper. Mix well to coat the chicken and potatoes evenly.
In a separate bowl, whisk together the milk and sour cream until smooth. Pour this mixture over the chicken and potatoes, stirring to combine.
If using, fold in the frozen mixed vegetables. Spread the mixture evenly in the prepared baking dish.
Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and sprinkle the shredded cheddar cheese on top. Bake for an additional 15-20 minutes, or until the chicken is cooked through and the potatoes are tender.
Let the dish cool for 5 minutes before serving. Garnish with chopped green onions if desired.