How to Make It
Make the meatballs In a large bowl, mix together the beef, egg, breadcrumbs and onion. Combine the ketchup and grape jelly in a large saucepan and bring to a bare simmer. Form the beef mixture into 3/4-inch meatballs and place each one in the ketchup sauce. Simmer the meatballs until cooked through, 15 to 20 minutes.
Make the spanakopita Preheat the oven to 375°. In a skillet, heat the butter over medium-high heat until foaming. Add the spinach and garlic and cook, stirring frequently, until the spinach is wilted, about 4 minutes. Remove from heat and let cool slightly. Transfer the cooked spinach to a fine-mesh sieve and, using a spoon, press out as much liquid as possible. Place the spinach in a large bowl, add the feta and dill and stir gently until mixed; set aside.
On a well-floured work surface, roll out the puff pastry to slightly large than its original size. Cut out 5-inch squares of pastry and place 2 tablespoons of the spinach mixture in one corner of each pastry square, leaving 1/2-inch of space around the edges. Brush the edges of each pastry with water, then fold the pastry diagonally to create a triangle of stuffed dough. Crimp the edges with a fork and brush the tops with the egg. Transfer the pastry to a baking sheet and bake for 15 to 20 minutes, or until browned and crisp.
Make the deviled eggs Slice the hard-boiled eggs in half lengthwise and place the yolks in a medium bowl. Add the mayonnaise, vinegar, mustard, onion powder and celery salt. Stir with a fork until well combined. If desired, place the filling in a pastry bag and pipe the mixture into the egg white halves; otherwise, spoon the filling into the eggs. Dust each egg with paprika.
Assemble the sandwich Cut the bread in half lengthwise and scoop out some of the inside to make room for the hors d’œuvres. Fill the bottom of the loaf with meatballs, followed by a layer of the spanakopita and a layer of deviled eggs. Add any optional toppings. Top with remaining half of the loaf, cut into 4 large slices and serve.