Heat oil on a griddle or in a large skillet over medium-high heat.
Add the ground beef and cook until fully browned, breaking it up with a spatula as it cooks. Drain any excess fat if needed.
Stir in the diced bell pepper, onion, and celery. Cook for about 3 to 5 minutes, or until the vegetables have softened.
Sprinkle in the Cajun seasoning, garlic powder, salt, and black pepper. Stir everything together so the seasonings are evenly distributed.
In a separate bowl, mix together the ketchup, mustard, tomato paste, Worcestershire sauce, and hot sauce.
Pour the sauce over the beef and vegetable mixture, stirring well to coat all the ingredients. Let it simmer for about 3 to 5 minutes until the sauce thickens and everything is heated through.
Lower the heat on the griddle or use a clean pan to toast the buns. Spread butter or drizzle oil directly on the hot surface.
Place the burger buns cut-side down on the buttered surface and toast until golden brown, about 1 to 2 minutes.
Scoop a generous amount of the sloppy joe mixture onto each bottom bun.
Add a slice of cheese if desired, then top with a handful of French fried onions for crunch.
Place the top bun over the filling and serve immediately while hot.