Strip the meat off the carcass and set it aside. Pop the chicken bones in a big pot with a peeled onion, a peeled carrot, and a stick of celery. Simmer until you have a rich stock, then sieve and get rid of the bones & boiled veg.
Then add the reserved chicken meat, your leftover parsnips and potatoes, and possibly a few more potatoes/carrots/celery. If you'd like an English-style soup, throw in a few herbs (thyme, parsley, etc is quite nice). Cook until raw veg is pleasantly tender & taste for seasoning.
If you like the idea of a curried chicken soup, you can also fry up a bit of curry powder in some oil and dump it into your soup along with some sliced garlic, then squeeze in some fresh lemon juice at the very end. (I did this a couple of weeks ago and it was a winner.)
At this very moment I'm boiling my chicken bits up with left over roast potatoes and leeks all together. I've already boiled the carcass and picked it to make a stock along with the juices saved from the roasting tin, then I'll blend it once everything has boiled up. I'm also making some fresh bread and I'm really looking forward to dinner
Add sliced ginger, a few salad onions sliced lengthwise and simmer 15 min while you slice up some veg - I like shitake mushrooms, bok choy, and napa cabbage. You can cube some tofu as well if you have it.
Add the veg, a couple of glugs of soy sauce, and simmer until the veg is tender, then add egg or glass noodles, the reserved chicken, a handful of salad onions sliced into rings and a drizzle of sesame oil.