Position the racks in the upper and lower thirds of the oven and preheat the oven to 400F.

Make the squash filling: Put the squash on a rimmed baking sheet and drizzle with the olive oil. Toss to coat the squash with the oil and then spread it into an even layer. Roast until very tender and browned around the edges, about 20-25 minutes, making sure to not let the squash roast to the point of drying out. Remove from the oven, transfer to a medium bowl, and let cool slightly. 

While the squash is roasting, add the beef, onion, salt, and pepper toi a bowl and mix well to combine. Refrigerate until ready to use.

When the squash is cool enough to handle, mash it with a fork to a smooth consistency (it's fine if it's a little bit chunky). Stir in the onion, salt, and pepper. Taste and adjust the salt and pepper if necessary — the filling should be seasoned well.

Line two baking sheets with parchment paper. Lightly dust your work surface with flour and lay out 16 of the puff pastry squares. Or, if using larger sheets of pastry, roll one sheet out into a 12-inch square and cut it into sixteen 3-inch squares.

Place about 1 tablespoon of the squash filling on the center of each square. Bring the top two corners of the square downward to the center so they meet and cover most of the filling. Bring the lower part of the pastry square up and over the filling, almost like you're making a triangular paper hat. Wrap any extending pastry points around the samsa to make a nice tight package and pinch the seams to seal.

Place the samsa seam side down on one of the baking sheets, leaving about 1-inch between them. Brush the samsa with the beaten egg.

Repeat with the remaining puff pastry and the beef filling to make 16 more samsa. You may have a bit of beef filling left over, depending on how much fat you needed to trim off the meat. Arrange them on the second baking sheet and brush with the egg.

Reduce the oven temperature to 375F. Place the baking sheets into the oven and bake the samsa for about 15 minutes, then rotate the baking sheets top to bottom and front to back and continue baking until the samsa are nicely browned all over, 25-30 minutes.

Transfer the samsa to a wire rack and let them cool for at least a few minutes. Serve hot or warm. Leftover samsa can be stored in an airtight container in the refrigerator for 4 days.