We usually put the onion soup mix IN the burgers, but I wonder if it wouldn't be a good substitute for the beef juice AROUND the burgers while waiting to be served?
Putting the mix IN the burgers incorporates the flavor throughout the meat.
Just adding it to some water? AROUND the burgers? will only coat the outsides. The onion flavor will not penetrate to the center of an already cooked burger.
The idea of the juice is to keep the burgers from drying out, not to flavor the meat. Using au jus, broth, even coke does add to the topical flavor soooo much better than keeping your burgers warm in plain water.
I love onion-soup mix burgers. Please keep adding the mix to the meat before cooking.
Adding a pack of the mix to whatever juice you decide to keep the burgers warm in, it will enhance, incorporate and bring out the flavor of the burger.