In a large mixing bowl, combine ground meat, finely grated onion, minced garlic, egg, breadcrumbs, milk, salt, pepper, and dried oregano. Mix well until all ingredients are evenly incorporated. Then, form the mixture into small meatballs, about the size of a golf ball.

Heat olive oil in a large skillet over medium heat. Add the meatballs in batches, searing them for 3–4 minutes on each side until browned.

Wash and slice medium potatoes evenly. Toss with olive oil, paprika, salt, and pepper until well coated.

Preheat your oven to 375°F (190°C). Grease a baking dish. Layer half of the seasoned potatoes in the dish, followed by the browned meatballs, then the remaining potatoes, and top with mozzarella and cheddar cheese.

Cover with aluminum foil and bake for 35–40 minutes until the potatoes are fork-tender. Remove foil and bake for an additional 10 minutes until cheese is golden.

Allow the casserole to rest for 5–10 minutes before serving. Garnish with fresh parsley if desired.