How does one glue together fried onions with an edible, savory adhesive?
Oooh! This is interesting! I think the first thing I'd like to know is there a requirement for the entire bar to savory? Because a lot of foods mix flavors and textures. Salted Caramel with nuts is an easy example. I can't remember the episode but there was an interview with David Chang of Momofuku on the Splendid Table where he spoke about his grand unified theory of flavor where the most satisfying dishes contain sweet, bitter, salty &amp; sour (often now with hot and umami added in). If you are willing to add in others you expand the range quite a bit as most sticky things are sweet. And you could have just that binding layer be sweet.
If not the next question I'd have centers around what sort of textures are acceptable along with how long until consumption? If, it short term maybe something  Tamagoyaki would work. That's more adhesion by pressing into a softer material and obviously wouldn't work if you are thinking bar as in a packaged bar that gets sold in a store.
Modified corn starch and rice are options. You could enrobe or possibly even spray those.