Quick Demi Glace?

Cook's Illustrated has a short cut recipe for demi. They use it in a Steak Diane recipe.

Agree with the others that, unless you're buying ready made demi, there's no such thing as a quick version.

If you've never made it before, though, I highly recommend going through the process. It's not all that labour-intensive or ingredient-intensive, just elapsed-time-intensive. It freezes very well so you can make it a couple times a year and have plenty on hand to add to soups, stews, sauces as needed for body and flavour.

Call around to some quality delis - they might have some. Our local market soup shop (like the guy in Seinfeld ) has Demi glaze in the freezer.
You cannot boil down some soup stock or conssume because it is all salt and not the real deal.

On the other hand, toss some Marsala in with the au jus and only you will know.

Demi-glace is a reduction combining brown sauce with veal stock. It needs to reduce by half. Unless you buy the stock already made there really isn't a short cut.