I got the idea from Heston Blumenthal, who uses the onion water to make a fluid gel. That was my original exposure to the SV caramelized onion technique. His recipe includes no salt and goes for 96hours at 85C. I think the addition of salt improves yield, but that may just be me imagining things. You'll definitely want to seal up your cooking vessel, as a lot of water will get lost due to evaporation at that time/temp.
The first time I made the recipe, I was struck by how much it made my kitchen smell like French onion soup... and it was only a short leap to try making it with SV onion water. I use yellow and white onions as well as shallots. Like Mokapot mentions above, sweet onions are best avoided because they're too, well, sweet. Haven't tried it with garlic water, but I keep meaning to. I've only made it twice. It's a fun idea and worth trying. I have no pictures of the soup, but will document the next time I make it (probably not until fall or winter). I do have some documentation from the fluid-gel dish though.
Here's the onion water in the bag:
Set with agar.
And on the plate (on both sides covered with toasted sesame seeds)
Hanger steak with bok choy, dashi braised daikon, and enoki mushrooms. Literally everything was cooked SV. I was going through a phase, to put things mildly.
