1. Preheat your oven to 375°F (190°C).
2. In a skillet, cook the ground beef and diced onion over medium heat until browned; drain excess fat.
3. Stir in garlic powder, onion powder, ketchup, and mayonnaise; cook for 2 minutes and remove from heat.
4. Spread the mashed potatoes evenly into a greased 9×13 baking dish.
5. Layer the beef mixture over the mashed potatoes.
6. Sprinkle shredded cheddar cheese over the top and garnish with pickle slices.
7. Bake for 15-20 minutes, or until the cheese is melted and bubbly.
8. Drizzle with additional ketchup before serving, if desired.