Beef and Egg Noodle Soup

butter

medium yellow onion (finely diced)

inch carrots (halved lengthwise, peeled and sliced into 1/2 pieces)

inch stalks of celery (sliced into 1/2 pieces)

leftover cooked beef (cut into large chunks (such as pot roast))

beef broth

egg noodles (such as pappardelle)

kosher salt and pepper

chopped parsley