*Place the chicken breasts in a Dutch oven or other large pot and cover with water to about 2 inches over the chicken. Bring the water to a boil over high heat and then reduce the heat to medium. Cook over medium heat for 15-20 minutes (depending on the thickness of your chicken breasts – mine are generally done at 15, so check one then) until chicken is cooked through. Remove the chicken breasts to a cutting board. Discard the water from the pot and rinse and wipe out the pot to use again. Chop the chicken into small, bite-size pieces and set aside. (*If you already have cooked chicken breasts on hand you can skip this step and use a pound of cooked chicken)
Melt the butter in the pot over medium heat. Add the chopped onion and cook for a few minutes until the onion is softened.
Whisk in the flour, salt and pepper until combined with the butter and onions and continue to whisk for another 1-2 minutes. Slowly whisk in the milk until combined and smooth. Add the water and bouillon base (or broth) and poultry seasoning and whisk in to combine.
Increase the heat to med-high and stir occasionally until boiling. Reduce the heat to med-low and add the egg noodles. Cook for 8 minutes, stirring regularly to prevent sticking. If you need a little more liquid toward the end you can add a bit more water or broth.
Add the chopped chicken, corn, and peas and carrots and stir until thoroughly combined. Cook for another few minutes until all ingredients are heated through.