You can use au jus in place of demi-glace, but the flavor will be slightly different. Au jus is made with beef stock and usually has a milder flavor and is thinner than demi-glace, which is made with veal or beef stock and has a richer flavor. If you want to use au jus instead of demi-glace, it becomes a thin broth, and the flavor is different. For me, au jus is not the best substitute for demi-glace. Check other like condensed beef stock, rich brown sauce, and more I mentioned above.