Start by preheating your oven to 375°F so it's fully hot by the time your mummy is ready to bake. Line a baking sheet with parchment paper to prevent sticking and make cleanup simple. This little step saves you from scrubbing a stuck pan later, and I always do it without fail.
In a large bowl, combine the ground beef, breadcrumbs, egg, milk, onion, garlic, Worcestershire sauce, a couple tablespoons of ketchup, salt, pepper, and smoked paprika. Mix gently with your hands just until combined, since overworking the meat makes it tough. I always stop as soon as everything looks evenly blended, not a second longer.
Transfer the mixture onto your prepared baking sheet and shape it into a long, slightly flattened oval, like a traditional loaf shape. Try to keep the thickness even so it cooks through at the same rate everywhere. This is also the moment to make sure your loaf isn't too thick, since a slimmer shape helps it cook faster and more evenly.
Roll your thawed puff pastry out slightly on a floured surface, then use a knife or pizza cutter to slice it into long, thin strips. Aim for strips about half an inch wide so they drape nicely over the loaf. I like uneven widths here and there, since it actually makes the mummy look more authentic.
Lay the pastry strips diagonally across the meatloaf, overlapping slightly and leaving a small gap near one end for the "face." Tuck the ends of each strip gently under the loaf so they don't unravel while baking. Don't stress about perfection, mummies are supposed to look a little messy and wrapped in a hurry.
Brush the beaten egg over all of the pastry strips using your pastry brush. This step is what gives the finished mummy that shiny, golden-brown look once it's out of the oven. Be gentle so you don't disturb or shift your carefully placed strips.
Place the baking sheet in the oven and bake for about 40 to 45 minutes, checking the internal temperature toward the end. You're looking for the pastry to turn deep golden brown and the meatloaf to reach 160°F internally. If the pastry browns too quickly, loosely tent it with foil for the last few minutes.
Once it comes out of the oven, let the mummy meatloaf rest for about 5 minutes before adding the candy eyes or olive pieces into the gap you left earlier. Add a small drizzle of ketchup or sriracha near the face if you want that spooky, bloody touch. Slice and serve while everything is still warm and the pastry is at its flakiest.