Preheat your oven to 350°F (175°C).
In a large bowl, combine ground beef, breadcrumbs, diced onion, egg, garlic powder, salt, and pepper.
Mix the ingredients until well combined.
Press half of the meat mixture into the bottom of a greased casserole dish.
In a separate bowl, combine cooked macaroni with milk and half of the cheddar cheese.
Spread the macaroni mixture over the meat layer in the casserole dish.
Top with the remaining meat mixture, pressing it down gently.
Sprinkle the remaining cheddar cheese on top.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes, until the cheese is bubbly and golden.