Preheat the oven to 375 degrees F.
Whisk together the soy sauce, hoisin, dark brown sugar, rice wine vinegar, sesame oil, garlic and ginger until the sugar dissolves.
Spread the sliced onions in an even layer across a half-sheet pan. Arrange the short ribs on top in a single layer.
Pour the sauce over the ribs and onions. Massage the sauce into the meat with your hands, flipping each rib so both sides are coated.
Roast for 20 minutes.
Flip the ribs and pour the beef broth around the edges of the pan, over the onions. Roast another 20 minutes.
Switch the oven to low broil and broil for 10 minutes, until the tops are caramelized and the edges are dark. Watch closely.
Rest 5 minutes. Serve with the onions and pan sauce spooned over the top and scallions scattered on.