Hmm, it has been a while since I've had lamb's quarters, so I'm working from stale memories. And we've largely weeded it out (well, depleted the seed bank by eating it young) in our container gardens. But in a couple of days we'll have our first CSA pickup and I'm bound to find plenty of it in the fields there to refresh my memory. :-) The sourness I remember isn't overt, though. It's more like what you get in spinach, which I guess makes sense.

Actually, what do you mean by sorrel? That's the name I use for a tasty little Oxalis that grows everywhere here, probably yellow woodsorrel (Oxalis stricta). Are you referring to Rumex acetosa, which I know as narrow-leaved dock?