By co-incidence, I had my first-ever sag aloo at Mrs Bari's Golden Temple restaurant on Friday, and it was one of the nicest sag aloos I have ever had. The only spice I could positively identify was mustard seed, but when I asked Mrs Bari what other spices she had used she said she did not know. Apparently Mr Bari looks after all the roasting/grinding/blending, and she just uses what he makes. The other point that emerged was that demand for aloo-based dishes is sufficiently low that she does not prepare them in advance. Instead, she peeled the potatoes when I placed my order, cooked them in a pressure cooker, and then made the dish from scratch. So unless she uses spices in her pressure cooker (a possibility, of course) then it looks as if her recipe requires no special pre-cook at all, and all the flavour is added when the dish is made.
** Phil.