Combine the water and the Worcestershire sauce and, using your hands, blend into the beef until evenly distributed.
Divide the meat into eight equal parts, roll into balls (but do not overwork the meat because ti will toughen your burgers), and flatten into discs about 1/2 inch thick and 4 1/2 to 5 inches in diameter. Chill for at least an hour or up to one day.
Place a cast-iron griddle on one area of a well-oiled charcoal or gas grill. Preheat all areas to high.
Combine the seasoning salt ingredients.
Combine all of the basting butter ingredients over medium heat and pour into a baking dish or disposable aluminum pan.
Swab the oil on the griddle, top with the onions, close the lid and cook until golden, about 3 minutes. Flip, close the lid, and cook for another 3 minutes.
If you have a grill press or a fire brick wrapped in heavy-duty aluminum foil, it is ideal to keep on hand to maximize the caramelization.
Brush the onions with the 2 tablespoons of butter, sprinkle with the pepper flakes and continue to cook until caramelized and tender. Transfer the onions to a bowl and cover to keep warm while the burgers cook. Do not clean the griddle.
It is preferable that the burgers be moved to a clean area of the griddle and grill evenly every time they are flipped. Depending on the griddle and the grill size, they will need to be cooked in batches to ensure there is a clean portion of the griddle and grill to flip to.
Season both sides of the burgers, using about half of the seasoning salt. Place 4 burgers on the exposed grate, keeping the other half clean to flip to. Close the lid, and cook the burgers until the meat easily separates fromt he grate and is well marked, about 2 minutes. Flip the burgers to the clean section, close the lid, and cook without moving them for 2 minutes more. Season the burgers with the remaining seasoning salt.
Transfer the burgers to the griddle, brush with the basting butter using the herb bundle, and continue to cook, flipping once (at this point the second 4 burgers can be started on the grate, see below) until you reach the desired doneness. Cook 2 to 3 minutes for rare, about 4 minutes for medium and 5 to 6 minutes for well-done.
Once the first 4 burgers and on the griddle, scrape the grates, re-oil, and repeat the grilling process. As the burgers reach the desired doneness on the griddle, transfer them to a sheet pan and cover with foil while the other burgers cook.
When all the burgers have been cooked, turn off the heat on all areas of the grill. Brush the buns with butter and top the burgers with cheese, if using. Close the lid for 1 to 2 minutes to melt the cheese and toast the buns.
Serve the burgers on the toasted buns.