I worked as a chef for a while (not in an Indian, I'm afraid). There used to be a massive vat of stock simmering all the time. It would go in all the sauces and soups we made. At the beginning of the week we'd have a huge delivery of bones and "chicken frames" from the butcher. We'd boil them up with vegetable trimmings. The whole thing was left simmering, even over night.

Few other tips on stock:
If the thought of boiling up carcases repulses you, you can now buy ready made "proper" stock at most supermarkets. I've never tried it though, so I can't vouch for its flavor.

If you roast the bones/meat first before simmering, you can avoid the worst of the smell (it also makes a more intensely savory stock as the bones caramelize).

Once a stock has come to the boil, just simmer on the gentlest of heats - this keeps it clearer.

Only use stock cubes as a last resort. Their usually comprised of fat salt and MSG

Stock freezes well so you can make a large amount, and freeze in batches.

I'm skeptical about this one. I don't think the savoriness of chicken stock tastes anything like the "missing taste". I'm not denying that some restaurants use it, I just doubt its the missing link.

I wonder if old gravy is just topped up with new batches. I knew someone who worked in a buffet style cafe. They served all day breakfasts and he claimed the baked bean trough was never cleaned out - it was just topped with fresh beans when it got a bit low. He said this was common practice.

I have often wondered why take away / restaurant curry can sometimes give you a dicky belly, while the ones cooked at home don't do this. I think it might well be that their gravy base could be hanging around for a few days, i.e fermenting.
I made a base on Monday, then left it until Wednesday morning before cooking a few curries. Though it hadn't started visibly fermenting flavor had noticeably improved (had a sour tang to it). I think its an important factor in the missing taste. Unfortunately, I didn't add enough oil to the gravy that there was enough to pour back into the base at the completion of each dish. I don't presently have a gas hob either, so I'm missing the flamed taste. Nevertheless, this is the closest I've been.
We know that most restaurants make their base sauce at least a day in advance. I can't believe they refrigerate it as the quantities are just too large. ( I doubt they'd be forthcoming in admitting this habit though, as it would flout food safety guidelines).

Some thoughts:
fermenting the base - that's something new!
It would rely on natural yeasts in the air, like naturally leavened bread.
If it's got a film of oil on top, it'll probably need stirring now and again to replenish the oxygen levels.
I'd be wary of fermenting anything with meat extracts, rice, or pulses in - the protein favors food poisoning bacterial growth (even if it's subsequently boiled, killing the bacteria, these bacteria can leave behind dangerous chemical compounds).
Finally, If restaurants are leaving their bases for a few days before use, they would probably do this at room temperature as the quantity would be difficult to refridgerate,

I wonder if the missing taste might come from the method of finishing the dishes over a high, flaming heat. When The oil catches it burns. This must alter the taste. Apparently, when barbecuing, the "barbecued" taste is achieved as much from the oil dripping from the food igniting as it is from the actual charcoal.
Has anyone here risked cooking a dish with the same ferocity they do in a restaurant kitchen?

This is the ingredient list I was given when I asked in  restaurant (unfortunately they didn't specify quantities, but I tried it with these amounts and the result was exactly the same to my taste).

3 parts tamarind pulp (strained)
1 part sugar
1part tom ketchup
1 part roast garlic (cloves roasted dry in their skins for 15-20 mins, then mashed with a fork)
half a part mint sauce
salt to taste

Combine all the ingredients
If you want it "authentic" red add a touch of coloring.

I have a large jar of home made mango pickle in by fridge - it's been there 18 months and still tastes fine (of course there is a lot of salt and oil in it - two very good preservatives)

Hi Pete,
Great thread (as usual). I was intrigued by what you said about them reusing the curry oil. It would be possible to replicate this on a smaller scale. Use more oil than you actually need for a dish. Once it's settled a bit pour the excess back into the jar with the rest of the curry oil. Do this with every curry you cook. Store the oil in the fridge between sessions.

I'll give it a go next time I have a cook up.

I had a suspicion curry houses might be doing something like this - it wouldn't be something they would advertise because, I imagine, the food hygiene people would be against it. There would also be a problem with vegetarian dishes. If a dhal or something had been cooked with oil that had been in contact with a lamb vindaloo, it wouldn't, strictly, be vegetarian.
I wonder if the other chefs you've seen neglected to show you this part of the process for the above reason?

Use the one in the sauce column this is the closest I have been to date. The only modifications I would make is once it's pureed cook it for another four hours, at least. I would also substitute the cinnamon for cassia bark as its flavor is more intense.

With the actual recipe itself, it might be worth adding the tom puree at an earlier stage, as indicated by some of the recent restaurant classes postings.

good luck.