Melt butter in a large pot over medium-low heat. Add the onion and a pinch of salt. Cook, stirring often until the onion is soft and translucent. About 8 minutes.
Add the broth and increase heat to high. Bring to a boil. Stir in the pasta, frozen veggies, and tomatoes and cook until the pasta is done, about 10 minutes.
Add in the chicken, cheese, lemon juice - if using, salt, pepper and green onions. Cook 3 minutes more to heat through.