I make curries a lot, and not just Indian. Thai, Burmese, Malaysisan, etc. and I always seem to have the same problem except when making coconut-based curries. Those seem to separate fine, as long as I use a coconut milk without added stabilizers such as guar gum. But anything onion or tomato based and I can't seem to get it to separate. Even if I cook the bajesus out of it. Any advice? I think the separated oil makes a very large difference in the scrumptiousness in the end.

I could not give you a scientific answer for it, because I am not a chef, but I find that when I am cooking a meat/poultry curry, the oil will not float to the top, until the meat is fully cooked. For a a lamb curry for example, I can tell by looking at it when it is done. The sauce looks all shiny with oil. At this point, if you let it rest, the oil will float to the top.

I do remove excessive oil, but not all. Because a lots of flavour is contained in it.

When cooking the curry sauce/paste, the oil doesn't always separate from the bulk a lot, unless you use as much amount of oil/butter/ghee as chefs do. What you do get is that the curry paste looks shiny and if you stop stirring it, you see a thin edge of oil around it.

Not sure what bajesus means.

"I think the separated oil makes a very large difference in the scrumptiousness in the end."

I agree totally :)

some of the problem may be due to natural emulsifiers in some of the ingredients. Capsicums contains them, so sweet peppers and chillies will hinder the release of oil. Also, the amount of oil makes a difference. And dare I suggest that the water content has to be reduced quite a bit for the water/oil emulsion to break.

Most of the flavour in a curry will be in the oil/ghee, so having it separated allows for the flavours to pass over the nose to be savoured.