Preheat your oven to 200°C (390°F). Line a baking sheet with parchment paper and keep your puff pastry cold and ready.
Cook the bacon in a skillet until crispy, then drain on paper towels and chop into small pieces. In the same skillet, caramelize the sliced onion until golden and sweet, then let it cool slightly.
Unfold the cold puff pastry sheet onto a lightly floured surface. Using a sharp knife or pizza cutter, cut the pastry into 24 small squares, about 4x4 cm (1.5x1.5 inches).
Arrange the pastry squares on the prepared baking sheet, spacing them about an inch apart. Keep the sheet chilled until you're ready to fill.
Place about a teaspoon of caramelized onion and a few bits of bacon onto the center of each square. Spread a small dab of Dijon mustard on top of the fillings.
Using a pastry brush, lightly brush the edges of each square with the beaten egg to give a shiny, golden finish when baked.
Sprinkle shredded cheese evenly over each filled square, then add a pinch of chopped herbs for fresh flavor. Optionally, top with sea salt or poppy seeds for crunch and visual appeal.
Bake in the preheated oven for 15-18 minutes, until the pastry is puffed, golden, and crispy, and the cheese is bubbling.
Once baked, remove the tray from the oven and let the bites rest for 5 minutes to set the fillings and crisp up further.
Serve these rustic bites warm, arranged on a platter, and enjoy the flaky, cheesy, savory goodness with your favorite drinks.
