The idea of using cream of mushroom soup for the sauce to bring it all together was considered, but I really thought we had enough flavours coming to the party, and the addition of mushroom soup would be unwelcome and confuse things.
I thought, "I can make a white sauce. That's easy."
So I picked up a bag of flour (I think it was about a kiloton for $1.25) at Albertsons on the way home, as we only had a tiny bit of practically fossilised flour left.
I thought, "I can make a roux. That's easy!", and then the sauce would be simple. I did not worry about the fact that a man missing a couple of fingers probably couldn't count high enough to count the years since I've actually made a roux, and I also didn't worry about looking up a recipe to remind me of quantities. Making a roux is easy, because you combine equal quantities (by mass) of flour and butter, so who needs a recipe?
Well, some time later I discovered that perhaps a bit over 6oz of each (plus a little extra flour to get the consistency right towards the end of the roux-making process, as one often has to do) was ... a lot. You know, a bit like space is big.
After I finished adding a litre and a bit of milk, I had at least 3 quarts of what appeared to be slightly thin sauce, but then that's ok because you simmer it for a bit. Well, during the simmering process it turned out that there was a bit of a gradient. The bottom was much more roux-like and the top was much more milk-like (one generally doesn't end up with 4" or so deep in the pan, so this doesn't normally occur -- one doesn't generally make a roux to serve sauce to 50-100 people either!), and by the time I'd simmered it for a little while and mixed it up well I had ........... about a litre and a half of something more roux than sauce!!! I had to transfer a portion of it to a smaller pan and turn it into something more sauce-like before adding the onions and shallots.
When it cooled a bit, the remaining 80-90% of my only mildly saucy (ur, sauce-ish) roux was a solid gelatinous blob filling the original pan.
We froze a bit over half of it in 2 ziplock containers, and put the rest in a larger ziplock container in the fridge.
I sense a lot of sauces in our future.