Preheat your oven: Start by preheating your oven to 350°F (175°C) to prepare for baking.
Prepare the meatloaf mixture: In a large mixing bowl, combine 1 pound of ground beef and 1 pound of ground turkey. Add 1 cup of breadcrumbs, 1 large egg, 1 medium finely chopped onion, 2 minced garlic cloves, ½ cup of ketchup, 1 teaspoon of salt, and ½ teaspoon of pepper. Mix well until the ingredients are thoroughly combined.
Shape the meatloaf: Transfer the meat mixture to a greased 9x13-inch casserole dish. Spread it evenly across the bottom, pressing down gently to form a compact layer.
Cook the meatloaf: Place the casserole dish in the preheated oven and bake for about 30 minutes, or until the meatloaf is cooked through and reaches an internal temperature of 160°F (70°C).
Prepare the potatoes: While the meatloaf is baking, peel and dice 4 large potatoes. Boil them in a pot of salted water until tender, approximately 15-20 minutes. Drain the water and return the potatoes to the pot.
Add sour cream and cheese: To the drained potatoes, mix in ½ cup of sour cream and 1 cup of shredded cheddar cheese. Mash the potatoes until creamy and smooth. Season with additional salt and pepper to taste.
Assemble the casserole: Once the meatloaf is done, remove it from the oven. Spoon the mashed potato mixture over the meatloaf layer, spreading it evenly.
Top with cheese: Sprinkle the remaining 1 cup of shredded cheddar cheese on top of the mashed potatoes.
Bake the final casserole: Return the assembled casserole to the oven and bake for an additional 20-25 minutes, or until the cheese is melted and bubbly.
Garnish and serve: Once out of the oven, let the casserole cool for a few minutes. Garnish with ¼ cup of chopped green onions before serving. Enjoy your cheesy and comforting dish with your favorite sides!