I had some of the soup for lunch today. I added salt and pepper, and just a small spoon of honey. Made a big difference. I heated it in the M/W and it was plenty thick, certainly didn't need any flour or arrowroot. I just added the grated Gruyère cheese on top and toasted a half a sourdough bagel with margarine on it separately. I'm not a butter fan, I hate milk and butter, it gives me the hebejeebes.
Anyhow I think it was right tasting so it will probably go into the regular menu items we eat. I think the slow cooker idea is great/easy and it makes a lot, enough for a month or more for the 2 of us. And it's not a heavy meat meal.
Yes, I've noticed most restaurant onion soup is pretty salty, but like I say I generally don't use salt. I'm not sure what I'm going to do with these huge ceramic 'onion soup bowls'. Probably into the black hole of storage, as if we don't have enough.