Secret Santa, I did forget one thing, as the onions are frying slowly, the onions will shrink in size and the oil will get deeper.
After the caramelized onions have given up their oil and are just brown and limp, I empty everything into a blender and normally use some of the liquid with mustard or peanut oil to start a strong curry.

Jerry

Stephan, I use mostly grapeseed or peanut oil, about a 3-4 tablespoons to start the chopped onions with, without overcrowding the onions, I use 3 med onions.
The oil doesnt evaporate much so the grapeseed oil gets infused with the onion oil, when the chopped onion dissapears to almost nothing pour into the curry base or into a bowl, then start another batch, and repeat until you get tired of keeping a eye on it.
Just dont let it burn or as you already know, there will be a bitter taste.

Jerry Griffiths

I met my first curry back in a small town in Wales back in in 1963, Ha, the Beatles, everything seemed simpler back then.
The first Pakistani restaurant (The Bagladesh) in Merthyr Tydfil , Wales, was a great new experience for me, a 17 year old apprentice stonemason and his best pal.
I remember going there after the last call at the pub (used to be 10:00 pm) and every Friday night challenging other inexperienced assorted work buddies telling them they couldn't dare eat a Vindaloo curry, remember, they didn't even know what a curry was.

Well their beer induced pride got the better of them and it ended up as a bet, if they couldn't eat the same curry as we ordered, they will foot the bill, if they could eat it, we would pay up.
By now I was friendly with the waiter and would wink to him as I ordered for our table, he winked back, he knew the preordained sign and ordered a normal Vindaloo for me and my accomplice and a triple Vindaloo for our unsuspecting guests, by the way, the chefs always enjoyed watching our victims suffer, peeking around from the kitchen door.

The triple Vindaloo had 3 times the hot of a normal Vindaloo and you can probably guess the result, after 1/2 hour of nose runs, eye watering and the occasional sob they gave up and paid the bill.
OK we were charlatans, tricksters if you will, but we were 17 year old apprentices earning 2 pounds, 50 shillings a week and they earned 4 times our pay.  I don't have any guilt, I figure they were just happy to be introduced to something other than a fish and chip night, so there.

The chef was a friendly man and walked me around the kitchen one mid week night.
I only remember the huge pot of simmering base.
He put a ladle into it and I saw the familiar red oil floating on top, mix downward with the yellow cats eye colored mixture below.

I live in beautiful California now, the (not top class, just local) Indian restaurants I have ate at and talked with the owners, just add more curry powder to make different curry, (i.e) 2 tsp of powder for a Korma= 3 for a Madras= get the idea!
They say the American customers don't know the difference, they are probably right, but I cook curry my own for our family at least twice a week, my family are getting to be curry experts.

I am a ex-patriot curry head and have tried to make that same Vindaloo curry taste all of the rest of my life, but have never got close until I tried this:

I medium chopped 4 yellow onions, put them in peanut or grapeseed oil and caramelized them until they melt (whole pieces, no blender, that just makes a onion puree and no oil) and then all is left is the onion oil (not a lot at a time or they will just boil) and just kept taking the onion oil out and adding more onions to the pan until they melted away (be patient with very low heat) and I ended up with a whole 1/4 pint of onion oil, this will probably take 3-4 hours, you can adjust the amount of onions just to to test.
I added the usual generic base to a curry and added 5 Tbls of the onion oil, it gave me the old taste I remember, it may not work for you, but I don't see this in any other posts, maybe this is the taste you old guys remember, or maybe??

I am getting ready to make a homemade tandoor oven as the websites show out there.

I live in Somis, California, my business website is www.venturacaststone.com