Preheat your oven to 350°F (175°C).
In a large skillet, sauté onions and garlic until translucent.
In a large mixing bowl, combine the cooked corned beef, diced potatoes, sauerkraut, sautéed onions, garlic, sour cream, mustard powder, black pepper, and smoked paprika.
Transfer the mixture into a greased baking dish, spreading it evenly.
Top the casserole with shredded Swiss cheese.
Cover with foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is golden.
Let it rest for 10 minutes before serving.