Preheat the oven to 375℉. Line a baking sheet with parchment paper and set aside.
Add your diced onion to a frying pan with ½ tbsp of olive oil and cook until translucent and tender, about 5 minutes. Remove from the pan and place in a bowl.
Cook the chopped bacon in the same frying pan until browned and crispy. Remove from pan onto a paper towel-lined plate. Once cooled a bit, mix together with the cooked onions.
Gently open the puff pastry sheet on a sheet of parchment paper or a lightly floured surface, and roll it out a few times with a rolling pin. Cut the pastry sheet into 4 even squares with a sharp knife. Place the squares on the prepared baking sheet.
Spread 1 tsp of dijon mustard onto the bottom of each square, making sure not to go all the way to the edges. Evenly distribute the bacon/onion mixture and the shredded cheese into the middle of each puff pastry section.
Lightly beat the egg in a bowl, and use a brush or your finger to brush the edges of each puff pastry. Fold one corner of the puff pastry to the opposite/diagonal corner, creating a triangle. Use a fork to crimp the edges of the turnover and seal it.
Brush the top of each turnover with the egg wash.
Bake in the oven for 17-20 minutes, or until the puff pastry is golden brown and cooked through. Allow the pastries to cool for a few minutes before serving while still warm.
Notes
Make sure you let your puff pastry thaw before starting the recipe.
Don’t forget to seal the edges with a fork, otherwise the filling might seep out when in the oven.
Try not to overfill the turnovers, if there is too much filling is might leak out in the oven.
Freshly grated cheese melts better than the bagged shredded cheese, so shred up your own if possible.