Plain yoghurt/greek yoghurt. Juice of about half a lemon (the acid helps tenderise the meat), and yes finely chopped onion (about half an onion depends on how much meat you have.

Another tip... cook it in the morning, then reheat it later on. Gives the flavours time to do their stuff. Ever wondered why takeaway seems to taste better cold the morning after?

Ghee is simply clarified butter/veg fat, with all the moisture taken out, and has a slightly higher smoking point.