Preheat oven to 350°F.

Using a large skillet, add olive oil and place over medium-high heat. Once hot, add onion, celery and bell pepper; cook for 5-6 minutes, stirring often. Add minced garlic and cook for 1-2 more minutes.

Using a large bowl, add onion mixture, ground beef, Worcestershire sauce, breadcrumbs, ketchup, Cajun seasoning and eggs; stir until just combined.

Spray a standard loaf pan with nonstick baking spray. Transfer meatloaf mixture into pan. Bake at 350°F for 60 minutes.

Using a small mixing bowl, combine all Sauce ingredients (ketchup, mustard, brown sugar and Worcestershire sauce). Remove meatloaf pan from oven and pour off any excess grease. Spread sauce mixture evenly on top of meatloaf and continue baking for 15-20 more minutes, or until internal temperature of meatloaf reaches 160°F.

Let meatloaf rest for 15 minutes before slicing.

Brush tops of bread with softened butter.

Using a griddle or large frying pan, place bread butter-side down. Cook over medium heat until bread is golden brown.

Meanwhile, combine the dijon mustard, ketchup and mayonnaise together in a small bowl.

To build sandwiches, brush a generous amount of sauce onto the non-buttered side of 4 pieces of bread. Add thick slice of meatloaf and a slice of mozzarella cheese to each sandwich.

{Optional} Place sandwiches open-faced under a broiler until cheese has melted.

Finish by adding crispy onions and lettuce. Top with remaining slices of buttered bread.