I love garlic too. You have to keep the base bland though remember!
I've given quite a bit of thought into the slow cooking of onions. I don't think they pre cook them in bulk. I just can't imagine a chef putting a spoonful of sweaty pre cooked onions into the pan - I don't imagine they'd sit well. Plus, we would have heard about it before now.
I think that firstly, not all mains need onions, and those that do don't all need them finely chopped and sweet. There are loads of Madras recipes I have tried and enjoyed that do not contain onions (other than those in the base obviously). So say for example a "curry", Madras, Vindaloo and Phall don't need onions - that's a fair whack of the meals already.
I'm sure they'd have time to give the onions 3-5 minutes in the rest of the dishes. Also bear in mind that if two people order a dish needing the onions done this way they'd probably just bang on two portions of the onions in one go - and so on.
This is definitely one question that you'd get away with asking when you're getting a curry.