this i feel is a very good question and worthy say of a poll to find out who is frying and who is boiling.
i also feel "bitterness" is being inadvertently used when "neutral" is possibly a better description.
all the bases i've made on every occasion (rajver, saffron, ifindforu, Ronnoc's, ivangoughs) don't taste bitter at all. they don't have an "underlying sweet tone" though.
if i make CTM for example the sweet tone does not matter - the CTM tastes as good as BIR for me.
if i make the more standard curries ie madras and compare it side by side with BIR/TA the BIR is clearly much sweeter.
the question for me is where and how do BIR's add this "underlying sweet tone" - is it:
A) - in the base by frying the onion
B) - at the cooking stage by using previously fried onions
C) - by adding some other ingredient in the base
D) - by adding some other ingredient at the cooking stage
