Heat your grill. Preheat your grill to medium-high heat - about 400°F.
Make the burger patties. Mix ground beef, Worcestershire, pepper, dried onion and salt in a small mixing bowl. Form into 3-4 burger patties.
Cook the burgers. Place the burger patties on the preheated grill and cook for 3-5 minutes per side, depending on the desired doneness. The internal temperature should be 160° on an instant-read thermometer.
Make the sauce. While the burgers are cooking mix the mayonnaise with the Worcestershire sauce. Refrigerate until ready to use.
Finish the burgers. Turn grill to low and top each burger with a slice of cheese. Place brioche bun halves sliced side down on the grill away from the heat. Close the grill and let buns toast and cheese melt for 1-2 minutes.
Assemble the burgers. Remove everything from the grill. To assemble the burgers, spread the bottom half of each roll with the mayo mixture. Top with a burger patty and then add the caramelized onions. Top with remaining roll half and serve immediately.
Notes
If you haven't previously made the caramelized onions, you'll need to add about 45 minute to your prep time.