Preheat the oven to 400F degrees and spray a 9x13-inch baking dish with nonstick cooking spray and set aside.
In a large skillet over medium high heat, melt 4 Tablespoons butter. Add in 1 small yellow onion, diced and cook for about 3-4 minutes, or until the onions are softened and translucent.
Next, whisk in 4 Tablespoons all purpose flour and keep stirring while it cooks for about 1-2 minutes.
Slowly pour 2 cups chicken stock or broth into the flour mixture (whisking constantly) along with ½ teaspoon onion powder, ½ teaspoon paprika, ¼ teaspoon garlic powder, ½ teaspoon dried thyme, 1 teaspoon kosher salt and ½ teaspoon ground black pepper.
You want to whisk to get out any lumps. Bring to a boil, then turn down the heat and allow the mixture to gently simmer. Whisk frequently for another 1-2 minutes until the mixture has thickened and is well combined.
Turn the heat all the way down to low then stir in 4 ounces (1/2 block) cream cheese, cubed and softened to room temperature and ¼ cup milk until well blended. Whisk constantly until the cream cheese has melted and is combined.
Turn off the heat then add in 10 ounce bag frozen mixed vegetables and 3 cups diced rotisserie chicken and stir to coat.
Spread the mixture into the bottom of the prepared baking dish.
Place 32 ounce bag frozen tater tots in a single layer on top of the chicken mixture (it's okay if you don't use them all). Bake in the preheated oven for about 35 minutes, or until the tater tots are cooked through and a golden brown color.
Finally, sprinkle 1 ½ cups shredded cheddar cheese shredded cheese evenly over the top of the tater tots.
Bake for another 4-5 minutes for the cheese to melt (or place under the broiler for 1-2 minutes for a crispier cheese topping.)
Garnish with a little parsley (optional) then serve.
Notes
Tater tots do not need to be defrosted for this.
If you don't like cream cheese (or you don't have it on hand), just leave it out. You don't need to replace it with anything. This just adds creaminess to the mixture.