How to Make It
Preheat a grill to medium (350°F to 400°F). Place ground chuck, scallions, Creole seasoning, and cayenne in a medium bowl. Using hands, gently combine. Shape mixture into 4 patties, and place on a plate. Clean bowl thoroughly. Stir together mayonnaise, parsley, mustard, hot sauce, and lemon juice in cleaned bowl; set aside.
Place patties on oiled grates. Grill, covered, until no longer pink, about 5 minutes per side. Remove from grill; loosely cover patties with aluminum foil. Place onion slices on grates; brush with oil. Grill, covered, until slightly charred and tender, about 3 minutes per side.
Spread 1 tablespoon of mayonnaise mixture on bottom half of each bun. Top each with romaine lettuce, 1 patty, and grilled onions. Cover with bun tops; serve.