Preheat the oven to 400 degrees and spray a nine-inch baking pan with cooking spray.
Put the chicken in a saucepan and cover it with water. Set the pan on the stove and bring the pot to a full boil. When the water is boiling, put the lid on the pot and remove it from the heat. Set a timer for 20 minutes.
When the time is up, use cooking tongs to remove the chicken from the water to a plate to cool.
Allow the chicken to cool enough so that you can handle it. When you can touch it without burning yourself, shred the chicken and put the pieces in the bottom of the prepared baking pan. Open the can of vegetables, drain the liquid in the veggies and stir this into the chicken. Set this aside.
Drain the cooking water from the saucepan and wipe it out with a paper towel.
Now you can make the sauce. In the pan, melt the butter over medium heat. When the butter is melted, whisk in the flour and cook this for about 2 minutes, whisking the whole time.
Slowly, whisk the chicken broth into the flour, adding the broth a little at a time. Take your time with this step to avoid making lumps in the sauce. Add the milk, and then let the mixture cook for a minute or two more to thicken, stirring constantly.
Add the parsley and onion powder, stirring to combine. Salt and pepper the mixture to taste.
Pour the sauce over the chicken and veggies in the baking pan and stir it to combine.
Now, move on to making the cornbread topping.
In a medium-sized bowl, stir together the cornmeal mixture and the sugar. Add the egg and the buttermilk, along with the cooking oil.
Use a spatula to scrape this mixture out on the top of the chicken-veggie and sauce mixture into the pan. Do not stir it. Just spread it over the top of the mixture in the pan.
Place the baking pan in the oven for 25 to 35 minutes or until it is lightly browned on top.
Remove the pan from the oven and spoon the casserole into dishes to serve. Store leftovers in the refrigerator tightly covered with a lid or with aluminum foil for up to three days. Leftovers can be reheated in the microwave.