Divide ground beef into four equal burger patties. Careful not to overwork the meat. Season all sides with garlic powder, salt, pepper and Worcestershire sauce.
Slice onion into quarter inch thick rings.
Heat grill to medium-high heat and lightly oil. Once the grill is hot, carefully add onion rings and shishito peppers (stems removed). Grill onions and peppers until they are somewhat tender and have nice grill marks. Remove from grill.
Cook burgers for about 4-6 minutes on each side depending on doneness level you prefer. Add sliced havarti cheese and melt.
Lightly toast buns cut side down on the grill.
Assemble burgers by spreading mayo and steak sauce on the toasty buns. Top with cheeseburger patty, grilled onions and shishito peppers. Enjoy!