Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).
In a large mixing bowl, combine ground beef, breadcrumbs, egg, milk, onion, garlic, salt, pepper, and oregano until fully incorporated.
Press the beef mixture into an even layer in a 9x13 inch baking dish.
Spread ketchup evenly over the surface of the meat.
Bake for 25 minutes then remove from oven to drain excess fat.
Distribute mixed vegetables in an even layer over the meatloaf base.
Spread mashed potatoes over the vegetables, sealing the edges to the side of the dish.
Top the potatoes with shredded cheddar cheese.
Return to oven and bake for 30-35 minutes until the internal temperature of the meat reaches 160 degrees Fahrenheit.
Allow the casserole to rest for 10 minutes before serving.