Preheat oven to 350°F degrees.
Cook ground beef in large skillet.
Add diced onion to beef mixture.
Season with salt and pepper.
Drain excess fat from pan.
Mix mayonnaise, ketchup, and mustard together.
Add pickle relish to sauce mixture.
Combine cooked rice with beef mixture.
Spread mixture in baking dish evenly.
Top with half the cheese layer.
Bake for 20 minutes until bubbly.
Add remaining cheese on top.
Bake 5 more minutes until melted.
Top with lettuce, tomatoes, and pickles.