I am actually a bit busy today and off line tomorrow (most of the day). We are having Christmas in July in August which my friend is cooking which means I have to come up with the recipes which means I will end up cooking
Did you follow all that?
I don't know about the hot and sour soup. I know about Thai hot and sour, which is called tom yum. At a guess, I would say the take away is using Thai tom yum as a base. The next question is are they making thier own or using a commercial product?
I would guess they are using a commercial tom yum paste. They are so good it's hard to tell them apart from home made. As for the "secret" ingredients I am guessing good old salt and pepper! Does the soup have a soy or fish sauce aroma to it? If not then they are using plain salt...
Hot and sour paste is used "to taste". Which means you use as much or as little as you like to your taste. Being take away, they would be going middle of the road. Because amount is user determined it does not have any seasoning. For example, if someone liked it really strong it would be to salty if it were pre-seasoned. See what I mean?
So the soup needs to be seasoned or it would be so bland you would not pay for it...Actually another thing I just thought of. They might be frying the tom yum paste. It is normal to add the paste directly to the water or stock to make the soup. But a very quick and hot fry first would release the flavours more. The paste is made "cold". It is blended or crushed into a paste, that is how it is made. A quick and hot fry would cause the cells of the ingredients to burst in the exact same way as they do when making a curry. That bursting would release all the oils from inside the cells and the flavour would be much more intense. They use oil such as peanut etc, to bind the ingredients together in the paste, so no need to add extra oil. Just drop the desired amount of paste into a hot wok for say 10 seconds maybe 15 seconds, then add the water (or stock). Then build the soup from there with the vege.
I hope that is of help