It's all in the stock.
I am forever grateful to the MNer who said about making stock in her slow cooker overnight, so 12 hours on low. The darkest, richest and most delicious stock I have ever made. I made a lovely stew with it.
But absolutely don't chuck out the fat that solidifies on top of the stock. Use it to fry your stew veg in, along with butter for flavour. Fat is flavour.
This is what I did:
For stock, fry carcass in knob of butter with chopped onion and carrot, generous s&p, L&P, mushroom ketchup, 3 bay leaves, bouquet garni, fresh thyme & rosemary until colouring. Then cover with boiling water and tip into slow cooker. Leave on low overnight, then strain, pick off meat, discard veg.
Use stock for the broth: Cubes/chunks of swede, celeriac, celery, onions, carrots (or other root veg of your choice) fried in butter with s&p, fresh thyme & rosemary until colouring, add half the leeks and cook a bit more. Add chicken stock and bubble til veg almost done, then mix in leftover chicken, rest of the leeks and shredded savoy. Put a lid on and cook for about 5 mins til all done. Stir in some cream, heat through, serve immediately.
It was divine.