Just being reading this post online,
I've no idea if this preparation of a curry base is commonly done in restaurants, but I can talk (more or less) about what some of the worse Indian restaurants in the UK allegedly do.
Years back,I skimmed through a book on (UK) Indian restaurant cooking. I think it was The Curry Secret by Kris Dhillon, though if more than one book has been written on the subject it could well have been a different book.
It was interesting in a horrifying sort of way. At the time I had recently moved to the UK from India, and had been mystified/disgusted by the food served in Indian restaurants in the town I was living - I really couldn't understand quite what they had been doing to make the food so bad (particularly because the restaurants included some places that actually smelt pretty good if you walked past them earlier in the day while onions and spices were being fried). This book went a long way in explaining how this really unpleasant food had been produced.
I certainly didn't read the book carefully, or at length, and it was about a decade ago, so some details may be wrong. But from what I do recall, the method for most 'slow-cooked meats' was more or less as follows:
Boil meat (or poultry, or whatever else) in water flavored with turmeric. When meat is done, discard water, shred meat and set aside for when dish is ordered.
Make a very large batch of generic 'curry sauce' by frying onions, ginger, garlic, tomatoes, and a few spices (no meat added at this point). When a specific dish is ordered, add more of certain ingredients to the sauce for that dish - e.g. sprinkle in large amounts of Kasoori methi (dried fenugreek), or add extra cream, etc. then add plain shredded meat, re-heat and serve.
This explained not just the bland tastelessness of most dishes served, but the strangeness of the many menu items:
for example many places had (or still have?) one whole section of the menu devoted to foods strongly flavored with dried fenugreek - though a popular enough spice in Indian it is not crumbled at random over virtually any dish in the manner offered by these restaurants.
Another menu section in one place that sticks particularly in my mind was a choice of 'South Indian' chicken, pork, lamb, or beef in a cream sauce with pineapple and flambeed brandy (no, I did not eat in that restaurant, I simply read its menu in the window, shuddered and moved on.)
Clearly, this type of flavoring is simply a ploy of using strong-tasting or distinctive ingredients to make one generic sauce taste different. It is also clearly different from the spice/onion base mentioned in C. Punjabi as in that case certain key ingredients are being prepared in bulk and then combined and cooked together with the meats.
(Of course, preparing a large batch of a few basic ingredients that are commmonly included is not necessarily a bad thing. Independently of cookbooks and simply for her own convenience my (Indian) mother-in-law has for years made a base by frying a large batch of onions, ginger, garlic, tomato, cumin, coriander and turmeric, which she then freezes in smaller portions and uses as a short-cut in her vegetable dishes. It works for her. Almost all her food is cooked in one particular regional style, and there is not usually a huge variation in the type of spicing she uses, so making up a large batch in advance makes sense for the way she cooks. )