Preheat the oven to 400°F and cut the butternut squash in half lengthwise. Remove the seeds and pulp from the inside and use a melon baller to scoop out some additional pieces of the squash to make room for the filling.
Melt the butter in a large frying pan and add in the diced onion. Cook the onions until they become translucent, then add in the rosemary, peas, and cubed pieces of bread. Cook everything together briefly. Remove the mixture from the heat and let it cool for 10 minutes, then stir in the egg.
Season the ground beef with paprika, salt, and pepper, then place it on a large piece of parchment paper. Place a second piece of parchment paper on top of the ground beef and use a cutting board to press down on it to mold it into the shape of a large, flat rectangle, then remove the second piece of parchment paper.
Cut the celery stalks so that they're slightly shorter than the long side of the ground beef rectangle. Fill the celery stalks with shredded cheese and place the stuffed celery stalks together so that the shredded cheese is sealed in the center. Place the celery stalks lengthwise near one edge of the ground beef rectangle.
Use the parchment paper to help roll the ground beef up around the stuffed celery stalks, then use your hands to seal in the edges.
Spoon the bread filling into the two hollowed-out halves of butternut squash, then place the ground beef roll in the center of one of the halves.
Press the two halves of butternut squash together, making sure the bread filling surrounds the ground beef roll, then place the stuffed butternut squash between the wells of a muffin pan. Transfer the muffin pan to the oven and bake the stuffed butternut squash at 400°F for 1 hour.
Slice into this fragrant dish and see how stunning this stuffed butternut squash is!