Yo Dudes,
My thought is (and i know its been said before) is that it is recycled oil from previous dishes. Now I don't think this is the end of it either since all the oil does is carry flavour from previous dishes. The flavour has to be there to start with BUT... I do think those flavours mellow with heat and time. The reason I suspect these things is manifold one being this: ever walk past an asian takeaway joint and wonder how those places get to smelling like the do? I rekon it's the repeatedly reaheated oil that has little bits of everything floating in it such as little bits of pastry from wantons and spring rolls etc plus all the flavours from these foods have leached into the oil and mellowd over time. I suspect the same thing happens with indian spices and other ingredients mellowing in hot oil over time. now the other important reason was this: one time I was carefully watching an indian chef cook my dinner and I saw him do something remarkable but thought nothing of it at the time. He would skim off excess oil (which also had a tiny bit of my meal in it) from my dish and put it into the little plastic container in front of him. This is the same little plastic container he originally dipped into to add oil to MY meal at the begging of making my madras and he also started the next dish (after mine) with this same oil. So in this restaraunt, the oil going into each dish had been, at some time, been in a whole miriad of other dishes and possibly in the onion sauce and red sauce pots as well. This would lead one to presume that this is where the secret is coming from and why the flavour is so difficult to acheive at home. But the recipe methodology and ingredients have to be there to begin with. All this theory is doing is taking the emphasis off this silly idea of a secret ingredient. I hope this is of some use to someone. my sugestion to anyone trying to prove my hypothesis, is to take a little bit of oil from every dish you make say a tablespoon or two per every serving and add it to the next dish you make and from that dish add a bit to the next dish and so on and so forth ; ) The secret could be this tiny taste of a hundred sucsessive curries!!! YUM thats gotta help