In a large bowl, combine the ground beef, grated onion, minced garlic, egg, breadcrumbs, and milk.
Add the salt, black pepper, and dried oregano or Italian seasoning. Mix with your hands until just combined, being careful not to overwork the meat.
Form the mixture into golf ball-sized meatballs, rolling them between your palms until smooth.
Preheat your oven (350°F/175°C is recommended).
In a separate bowl, toss the potato slices with olive oil, paprika, salt, and pepper until each slice is lightly coated.
Grease a baking dish and arrange half of the seasoned potato slices on the bottom.
Place the meatballs evenly across this potato layer.
Top with the remaining potatoes.
Sprinkle the shredded mozzarella and cheddar cheese over the top layer of potatoes.
Bake until the potatoes are tender, the meatballs are cooked through, and the cheese is golden and bubbly (approximately 40-45 minutes).
Let rest for about 5 minutes, then sprinkle with fresh parsley before serving.