Preheat and Prepare: Preheat your oven to 375°F (190°C). Prepare a 9x13-inch baking dish by applying a thin layer of grease and set it aside.
Make the Meatloaf Base: In a large mixing bowl, combine the ground beef, ground pork, chopped onion, breadcrumbs, milk, egg, Worcestershire sauce, salt, black pepper, and garlic powder. Be careful not to overmix to avoid a dense texture. Press the meat mixture evenly into the prepared baking dish, forming a flat, even layer.
Bake the Meatloaf: Bake the meatloaf in the preheated oven for 25-30 minutes, or until it is almost cooked through and the edges start to brown. Remove from the oven and carefully drain any excess grease if necessary.
Add the Mashed Potatoes: Spread the prepared mashed potatoes evenly over the partially baked meatloaf layer. You can use freshly made mashed potatoes or store-bought for convenience.
Top with Cheese and Bacon: Sprinkle the shredded cheddar cheese over the mashed potatoes. Then, add the cooked and crumbled bacon evenly on top of the cheese layer.
Final Bake: Return the casserole to the oven and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly, and the meatloaf is cooked through. The internal temperature of the meatloaf should reach 160°F (71°C).
Garnish and Serve: Remove the casserole from the oven and let it cool for a few minutes. Sprinkle with chopped green onions before serving. Slice and serve this hearty, cheesy, and satisfying dish with a side salad or steamed vegetables.