Thanks for clarifying those things BB.
As for me, for what it's worth, I believe the following are important to creating that BIR "taste" and "smell" (most of which are just reiterating other member's comments):
1. Essential:
a) Use lots of oil to fry the spices (whether they be whole, powdered or paste form)
It is evident that oil is the medium by which the flavours of the spices are carried and distributed. In my opinion, it is essential not to skimp on the oil. I suggest at least 75ml per individual portion. Any excess oil can be skimmed off, at the end of cooking, and used to make another curry. Smell and taste the oil. It should have elements of both the "taste" and the "smell" (and colour) of a typical BIR curry.
Of course, modern day "awareness" of the "harmful effects" of too much oil (as well as salt, artificial flavours, msg, etc) might lead some people to use less of it. That's probably OK for still producing decent tasting curries, but it won't be the same, in my opinion.
b) Use fresh (powdered) spices (i.e. those that have not been poorly stored or sitting around or ages)
Powdered spices lose their flavour quite quickly (i.e. within weeks, certainly months, even though properly stored). Make sure your spices are fresh in that they still have a powerful aroma. If they don't, bin them. The same goes for spice mixes and curry powders and curry pastes.
c) Use a decent curry base
Although debatable, the essence of a BIR curry, in my opinion, is in the curry base. Without a decent curry base you will always be struggling to create that BIR taste and smell. Having said that, a decent curry can, of course, be produced without using a curry base. It's effectively cooked, in situ, with the main curry. However, this is more an "authentic Indian" cooking style, rather than a BIR cooking style, and will no doubt lead to a different (that is not to say it's inferior) taste and smell. To my mind, this (i.e. the curry base) is where "a gap" (i.e. an "unknown") exists between home cooking (and curry bases available on this forum) and BIR cooking.
d) Use a decent sized pan, matched to your heat source, and don't overfill the pan
I believe it is important to match your pan (of whatever material or shape) to your heat source. It is important to impart enough heat to ensure that the ingredients fry rather than braise. Overloading your pan will result in the latter and the flavours of the spices (and other ingredients) will not fully develop. Provided this is done, I see no reason why multiple portions cannot be successfully produced (though BIRs generally produce single portions at a time)
e) Use plenty of salt and/or MSG
Again, use plenty of salt (and/or MSG) in both the base and the main dish. I'd suggest at least 0.5 tsp of salt in the main dish and maybe 0.25 tsp MSG. MSG may be a throw back to earlier BIR curries of course (before effects of these things on "health" became such an issue!)
The reason I personally place so much emphasis on the above factors is that, as SS pointed out, BIRs produce various rudimentary dishes (such as mushroom bhaji, aloo bhaji, etc) which are little more than the above combination, which, nevertheless, have the taste and the smell. What else can it be? Really?
2. Important:
a) Use plenty of spice
This seems to be a bit at odds with other members' opinions (i.e. "less is more"). However, spices are obviously important to creating the taste and smell of a curry. If you use plenty of oil, and recover the oil for future use (whether from the base or from the main curry), you will need to add extra spices to compensate for the flavour "lost" in that removed oil. To my mind, you can quite easily add several tsp of spices to an individual portion of curry (provided it doesn't become "gritty").
Ground coriander and cumin are the most important BIR spices, primarily for flavour, and tumeric (and paprika) primarily for colour and chilli primarily for colour and piquancy.
b) Use spiced oil
There is no doubt, in my mind, that using spiced oil (whether recovered from the curry base, or from the main dish, or specifically created, can significantly enhance the smell and taste of any curry.
c) Use plenty of garlic
There is no doubt, in my mind, that garlic is an essential element of the taste and smell of a typical BIR curry. My recollection (from those BIRs curries of yesteryear) is that garlic seeps from the pores of the skin for days afterwards! A word of caution though. I find that using lots of garlic with lots of lemon juice can result in a very sickly tasting curry. So I'd say rather skimp on the lemon juice.
Okay, I've droned on too long so I'll stop there!