British Indian Restaurant (BIR) style onion sauce is super easy to make but takes a long time so plan ahead.
This onion sauce is part of what makes do piazza curries so good. It can also be added to other curries for extra flavour.
The secret to success is in the slow cooking. I usually cook my onion sauce in a low oven over night. Doing this gives the sauce more depth. You can use the finished onion sauce exactly as it is when you take it out of the oven or blend it if you prefer smoother curries.
When you take the finished sauce out of the oven, the top will be slightly blackened. Try it. If you like the flavour, stir it in. If not, skim it off the top.