Preheat the oven to 350°F.
In a large mixing bowl, combine the chopped corned beef, sauerkraut, Swiss cheese, rye bread, Thousand Island dressing, garlic powder, onion powder, and black pepper. Mix well until all ingredients are evenly coated.
Transfer the mixture to a greased 9x13-inch baking dish, spreading it out evenly.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for about 5 minutes.
Garnish with sliced green onions before serving.