Preheat oven to 400°F. Put parchment paper on baking sheet and place bacon on it. Roast for 20 minutes or until bacon is crispy.
While bacon is cooking, heat large skillet over medium heat and add the butter. Once melted, add the sliced onions and salt. Cook and stir every 5 to 10 minutes until the onions are soft and deeply browned. It should take about 20 minutes total. Add 2 tablespoon water to deglaze the pan. Set aside.
Roll out sheet of puff pastry on a floured surface and divide into 6 equal pieces. Add a spoonful of onion, 1 slice of bacon and ¼ cup of cheese over each piece.
Fold one corner over the cheese, wet your finger and then fold over the second corner. Tightly pinch together. Place each on a parchment-lined baking sheet and brush with egg wash.
Bake at 400°F for 15-20 minutes or until golden brown.
Notes
Make sure to cook the bacon until it's crispy and fully cooked. Undercooked bacon can result in a chewy and unpleasant texture.
The cheese is a key component of these turnovers, so make sure to use a high-quality cheese that melts well and has a strong flavor.
It can be tempting to stuff the turnovers with as much filling as possible, but overfilling can cause the turnovers to come apart open during baking.
Allow the turnovers to cool for a few minutes after baking to prevent burning your mouth. Serve warm or at room temperature for the best flavor and texture.