I've posted here before about the use of two sauces for the base because I had heard that it was used in some restaurants, but as far as I know no one who has posted recipes from restaurants here has ever seen a 2 stage base sauce used. I have a feeling that this method may have been used in restaurants many years ago, but for convenience the '2 part' base sauce has now become a one pot affair and this might also explain why the different curries are all starting to taste the same.
If you look at Kris Dhillon's curry base, she uses a 2 part method quite similar to the one you have posted, but she then combines them to make the final base sauce. I still rate the Kris Dhillon base quite highly even though it is quite simple and lacks most of the extra veg we now seem to think a base sauce needs. If you have never used the 2 part method it is definitely worth a try. The tomato part is very interesting because it is essentially fried for a long time and it gives the tomatoes a very intense, very savoury, almost smokey flavour.
I've been looking for a recipe or ideas for the very mild veg curry that usually accompanies a Biryani and I think the veg curry in this recipe sounds like it might fit the bill, so there's another curry I need to make.
I also noticed on some menus that the bhuna is described as being made with a 'separate double strength sauce', now what's that all about?