In a large pot of boiling salted water, cook pasta al dente, according to package instructions.
Cook veggies:
Meanwhile, in a large saucepan, melt butter over medium-high heat. Add onion, celery, and poultry seasoning, and cook until onion is translucent about 4 minutes. Add flour and mix with the vegetables, stirring for 1 minute. Gradually add the broth, stirring constantly, then bring to a boil. Reduce heat and simmer 5 minutes, stirring throughout.
Combine:
Add the mixed vegetables and cooked chicken, and combine with sauce.
No poultry seasoning? No problem. Use ½ teaspoon dried sage and ¼ teaspoon dried thyme. Alternatively, use 1 teaspoon Italian seasoning.
If you don't have a rotisserie chicken or leftovers, pan-sear boneless chicken breast in the same skillet, remove it from the pan, and cook the tidbits left with the veggies for added flavor. Give the chicken a rough chop before stirring it into the veggies and sauce.