Preheat the oven to 400°F (200°C).

Lightly grease 2 baking sheets.

Prepare a bowl of boiling water.

Prepare a bowl of iced water.

Chop the chard if not already chopped.

Briefly submerge the chopped chard in boiling water for 5 seconds to wilt it.

Immediately transfer the wilted chard to the bowl of iced water to stop the cooking process.

Drain the chard thoroughly.

Dry the chard on paper towels.

In a large bowl, combine the drained chard, thawed mixed vegetables, grated Cheddar cheese, chopped onion, and chopped cooked bacon (if using).

Season the vegetable and cheese mixture to taste with salt and pepper.

Cut each sheet of thawed puff pastry into 4 squares.

Spoon approximately 2 tablespoons of the vegetable mixture onto the center of each pastry square.

Brush the edges of each pastry square with lightly beaten egg.

Fold each pastry square diagonally to form a triangle, enclosing the filling.

Use a fork to firmly seal the edges of each triangle, preventing the filling from escaping during baking.

Place the assembled turnovers onto the prepared baking sheets, leaving some space between each.

Brush the top of each triangle with the remaining beaten egg to promote browning.

Bake in the preheated oven for 15-20 minutes, or until the turnovers are golden brown and puffed up.

Remove the baked turnovers from the oven and let them cool slightly on the baking sheets.

Serve the cheese and vegetable turnovers warm with a side salad, if desired.