When you are finished simmering your corned beef, strain and reserve the broth. Broth may be reduced if it is too dilute. Taste and add additional salt & pepper if needed.
Add back some corned beef or if using broth frozen back from earlier in the year, extra shredded rotisserie chicken works well. Also add carrots and mushrooms.
Simmer until the vegetables begin to soften; then add cabbage. Cook for a minute or two before adding your noodles. Do not overcook the cabbage.
Serve immediately.