Something I have noticed from my attempts - 'less is more!
I started with bruce edwards advice and made a very basic gravy, even more basic than yours, and started making curries with it. My ingredients are not too dissimilar to yours onion and interestingly the resulting dish tastes more complex than it actually is. Every time I fancy a curry now I try adding one extra thing to check the difference.
No revelations yet from this method but I have discovered something. One evening I quickly made a curry for work the following day and it had the taste and aroma I have been searching for. It had it in bucket loads and I was very, very, very delighted. I even posted the recipe on here in the section for Pathia.
What still eludes me is where it came from, I followed the same recipe and process as before but something special happened that time because so far I have not been able to recreate it.
It has to be technique or temperature because the ingredients did not change at all.
Have you tried using lime instead of lemon. Better IMHO.
Good luck