Ingredients
11 ounces boneless short ribs, trimmed of any large pieces of fat
11 ounces sirloin steak, trimmed of any large pieces of fat
4 slices red onion, each 1/2 inch thick
Olive oil, for brushing
Coarse salt
2 ounces blue cheese, sliced
4 sesame-seed brioche buns
Equal parts Dijon mustard and whole-grain mustard, mixed together, for serving
Romaine-lettuce leaves, for serving
Thinly sliced tomatoes, for serving (optional)
Preparation
Cut both meats into small pieces. Grind through the large holes of a meat grinder. Combine in a bowl, then grind together through small holes of grinder. Divide into 4 equal parts (each about 5 1/2 ounces). Form into patties, each about 3 1/2 inches in diameter.
Preheat grill to medium-high. Skewer onion slices crosswise with presoaked skewers; brush both sides with oil. Season both sides of burgers with salt. Grill burgers 2 1/2 minutes a side for medium-rare, placing cheese on burgers in last minute. Transfer to a plate.
Grill onion until tender and lightly charred, about 4 minutes a side. Meanwhile, grill buns until toasted, 30 seconds. Sandwich burgers and onions (removed from skewers) on buns with mustard mixture, lettuce, and tomatoes.
