Step 1: Preheat the Oven

Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch casserole dish.

Step 2: Prepare the Meat Mixture

1. In a large mixing bowl, combine the ground beef, breadcrumbs, eggs, chopped onion, bell peppers, mushrooms, garlic, milk, Worcestershire sauce, salt, black pepper, Italian seasoning, and smoked paprika.
2. Mix until just combined (avoid overmixing, which can make the meatloaf tough).

Step 3: Assemble the Casserole

1. Spread the meat mixture evenly into the prepared casserole dish, pressing it down to form a flat, compact layer.
2. In a small bowl, mix the ketchup, brown sugar, Dijon mustard, and apple cider vinegar. Spread this sauce evenly over the top of the meat mixture.

Step 4: Bake the Casserole

1. Bake uncovered for 40-45 minutes, or until the meat reaches an internal temperature of 165°F (74°C).
2. Remove from the oven and drain any excess grease carefully.

Step 5: Add the Toppings

1. Sprinkle the shredded cheese evenly over the baked meatloaf base.
2. Add the crumbled bacon on top.
3. (Optional) If using mashed potatoes, spread them over the meat before adding cheese and bacon.

Step 6: Melt the Cheese & Serve

1. Return the casserole to the oven and bake for another 5-10 minutes, or until the cheese is melted and bubbly.
2. Remove from the oven, let it rest for 5 minutes, then garnish with chopped parsley.
3. Slice and serve warm!