Bring a large pot of water to a boil. Add the egg noodles and cook according to package directions. Drain and place in a large mixing bowl.
While the noodles are cooking, dice the onion and mince the garlic.
Heat the butter in a small skillet over medium heat. Cook the onion until softened, about 5 minutes. Add the garlic and cook an additional minute.
Add the onion and garlic to the bowl of noodles along with the chicken, soup, sour cream, peas and carrots, mustard, salt, and pepper. Stir to combine.
Spoon into a 9x13 baking dish. Bake for 15 minutes or until heated through.
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Notes
We use dried Amish egg noodles, but any variety of dried egg noodle will work, such as no yolk noodles. We like to use shredded rotisserie chicken in this recipe, but leftover turkey is also delicious. You can use any variety of frozen veggies you like, just be sure to thaw them first so that they cook in the allotted time. Canned vegetables also work well. My cream of chicken soup substitute is a great swap for the canned soup. You could also use cream of celery, cream of mushroom, or any variety of cream soup that sounds good.