01 -
Heat olive oil and butter in a large skillet over medium heat. Add sliced onion and cook, stirring occasionally, for 15 to 20 minutes until softened and golden.
02 -
Stir in balsamic vinegar and brown sugar. Cook for 5 minutes until onions become deeply caramelized and sticky. Add thyme, salt, and pepper. Let cool.
03 -
In a separate bowl, combine ground sausage with salt, pepper, and Dijon mustard if using. Mix until evenly distributed.
04 -
Preheat oven to 400°F. Line a baking sheet with parchment. Roll out puff pastry on a lightly floured surface and cut into 4 equal rectangles.
05 -
Distribute cooled caramelized onion evenly across each pastry rectangle. Top with sausage mixture, shaping into logs and leaving edges clear for sealing.
06 -
Fold the pastry over the sausage to enclose. Press edges to seal and place seam-side down on the prepared sheet. Brush tops with beaten egg.
07 -
Bake for 20 to 25 minutes until pastry is golden and crisp. Allow to cool briefly before serving.