Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large bowl, combine the ground beef, breadcrumbs, 1/2 cup milk, diced onion, minced garlic, egg, Worcestershire sauce, ketchup, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix gently until just combined. Do not overmix.
Step 3: Press the meatloaf mixture evenly into the bottom of the prepared baking dish.
Step 4: Bake the meatloaf layer for 20 minutes to allow some of the fat to render and the meatloaf to begin cooking.
Step 5: While the meatloaf bakes, place the quartered potatoes in a large pot and cover with cold, salted water. Bring to a boil and cook until fork-tender, about 15-20 minutes. Drain thoroughly.
Step 6: Return the drained potatoes to the pot. Add the warmed milk/cream, butter, sour cream, remaining 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mash until smooth.
Step 7: Fold in 1 cup of the shredded cheddar cheese, crumbled bacon, and most of the chopped green onions (reserve a small amount for garnish).
Step 8: Carefully remove the baking dish from the oven. Spread the mashed potato mixture evenly over the partially baked meatloaf layer.
Step 9: Sprinkle the remaining 1/2 cup of shredded cheddar cheese over the top of the potatoes.
Step 10: Return the casserole to the oven and bake for another 30-35 minutes, or until the topping is golden brown and bubbly and the meatloaf is cooked through (internal temperature of 160°F/71°C).
Step 11: Let the casserole rest for 5-10 minutes before serving. Garnish with the reserved green onions.