Without onion your task is very difficult. But with what you have, i'd mix the chicken with Pataks Kashmiri and fry with the garlic and a little stock, you could add a little cream and the a little apple, but I would finish with chopped coriander. The potato I would cube and par boil then toss in curry powder and fry with more garlic.

Is this any good?

I've never done a "designer curry", so I will leave that to others, but one comment :  you speak of the meat as being left-over roast chicken -- may I ask if this is breast, leg or both ?  The reason I ask is that it will be hard to infuse any curry flavour into pre-roast breast, but absolutely impossible to infuse it into pre-roast leg.  That being the case, my inclination is to suggest that you do indeed use your slow cooker, which will allow the "curry gravy" to remain in contact with the chicken for the maximum amount of time.  Then, when the meat starts to fall off the bone, remove the meat and carcase, separate the former from the latter, then transfer the gravy into a frying pan or wok and reduce it until (a) the oil comes out (if it has not already done so), and (b) you have the consistency you are aiming for. Then re-introduce the chicken, add chopped coriander stalks and serve.

** Phil.