Preheat your oven to 375°F (190°C).
In a large skillet, brown the ground beef and diced onion together over medium heat until the beef is no longer pink.
Drain excess fat from the skillet and reduce heat to low.
Stir in the diced potatoes, ketchup, mustard, garlic powder, salt, and pepper, mixing well.
Transfer the mixture to a greased casserole dish and top with shredded cheddar cheese.
Cover the casserole dish with aluminum foil and bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Let the casserole cool for a few minutes before serving, garnished with diced pickles if desired.