Preheat the electric smoker to 225°F. Mix the ground beef, breadcrumbs, egg, onion, Worcestershire sauce, ketchup, smoked paprika, salt, and black pepper in a bowl. Mix just until combined so the meatloaf stays tender.

Shape the mixture into a compact loaf about 8 inches long. Place it on a piece of foil or in a smoker-safe pan, then smoke until the center reaches 150°F, about 1 1/2 to 2 hours. Use a meat thermometer to check the thickest part.

Remove the meatloaf from the smoker and let it cool until just warm. Pat the surface dry if needed. A cooler, dry loaf is easier to wrap and helps prevent the pastry from becoming soggy.

Heat the oven to 400°F. Unfold the thawed puff pastry on a lightly floured surface and roll it just enough to fit around the loaf. Spread the Dijon mustard over the top and sides of the meatloaf, then place it in the center of the pastry.

Fold the pastry over the meatloaf, trimming any excess and pinching the seams closed. Set it seam-side down on a parchment-lined baking sheet. Brush the pastry all over with the beaten egg.

Bake until the pastry is puffed and deep golden, about 25 to 30 minutes. Check that the center of the meatloaf reaches 160°F. Rest for 10 minutes before slicing so the filling holds together.