Preheat your oven to 350°F (175°C).
In a large skillet, brown the ground beef with the diced onion and minced garlic over medium heat until the meat is fully cooked and the onion is translucent. Drain off any excess grease.
In a separate bowl, mix together the browned beef mixture, frozen diced potatoes, cream of celery soup, milk, salt, black pepper, smoked paprika, oregano, and Greek yogurt until well combined.
Transfer the mixture into a greased 9x13 inch baking dish, spreading it out evenly.
Top the casserole with a generous layer of shredded sharp cheddar cheese.
Cover the dish with foil and bake for 30 minutes.
Remove the foil and continue baking for another 15-20 minutes, or until the cheese is melted and bubbly with a golden hue.
Allow the dish to rest for about 5 minutes before serving. Optionally, sprinkle chopped chives over the top before serving.