i do love cbm's video - for me 100% spot on.
a small suggestion that the video reminded me on.
came from a few words by my very good friend bengali bob - his local use 3 onions in a dopiaza.
for a bhuna i'd never really got the taste right until i use this info but dropping the onions to 2 ie you don't want the hot fried crisp onion in bhuna.
so in short adapt recipe to include:
1) service onion ie chopped pre par boiled cooked
2) pre cooked caremilised onion ie "pre-cooked slow fried" or as BB says bhuna onion - very dark