SET UP: Prepare a grill or grill pan and heat to medium-high heat.
FORM THE PATTIES: Gently mix the ground beef, Cajun seasoning, and scallions. Form into four 1/2-inch-thick patties.
MIX THE REMOULADE: Combine the mayonnaise, mustard, hot sauce, parsley, and celery in a small bowl. Refrigerate until needed.
GRILL EVERYTHING: Grill the patties, flipping once, to desired doneness, about 4 minutes per side for medium. Brush the onion and green pepper with the oil and grill the vegetables until lightly charred and tender, about 5 minutes total.
SERVE: Toast the buns on the grill for a minute or so. Put a patty on the bottom bun and top with remoulade. Add some onion and bell pepper, then add the top bun.
Recipes from "The Chopped Cookbook: Use What you've Got to Cook Something Great" (c) Copyright 2014 by Television Food Network, G.P. By arrangement with Clarkson Potter/Publishers.