Warm a small skillet over medium heat and coat with cooking spray. Add ground beef to the skillet and cook until browned. Season then set aside.
Prepare all toppings and special sauce, then set both aside.
To assemble 4 servings, create a base of about 1-2 cups of romaine lettuce in 4 bowls. Top each with about 3oz ground beef, 1 tablespoon of sauce, a drizzle of mustard, and desired amount of toppings.