Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, brown the ground beef with the diced onion until beef is fully cooked. Drain excess fat.
Stir in minced garlic, salt, and pepper into the beef mixture. Cook for 1 minute until fragrant.
Transfer the beef mixture to the prepared baking dish and spread evenly.
Top the beef with shredded cheddar cheese. Bake for 15-20 minutes until the cheese is melted and bubbly.
While the beef bakes, prepare the sauce by whisking together mayonnaise, mustard, relish, vinegar, paprika, onion powder, and garlic powder in a small bowl.
Remove casserole from oven. Top with shredded iceberg lettuce and sliced pickles.
Drizzle the prepared sauce over the top before serving.