Caramelize the onions: Heat a skillet over medium-low. Add butter or oil, then sliced onions and a pinch of salt. Cook 20–30 minutes, stirring occasionally, until soft, golden, and jammy. Add balsamic and a touch of brown sugar if using. Let cool completely.

Prep the sausage filling: In a bowl, combine sausage meat, mustard, herbs, garlic (if using), and pepper. Mix gently until just combined. Avoid overworking.

Prepare the puff pastry: Lightly flour your surface. Roll the puff pastry into a rectangle about 10 x 12 inches. Cut in half lengthwise to make two long strips.

Assemble the rolls: Spread a thin line of caramelized onion down the center of each pastry strip, leaving edges clear. Shape the sausage mix into two long logs and place over the onions.

Seal and chill: Brush one long edge of each pastry strip with beaten egg. Roll the pastry over the sausage to seal, seam side down. Chill the logs in the fridge 10–15 minutes to firm up.

Cut and finish: Slice each log into bite-size pieces (about 1.5–2 inches). Brush tops with egg wash and sprinkle with poppy or sesame seeds if you like. Score lightly on top for steam release.

Preheat the air fryer: Heat to 375°F (190°C) for a few minutes. Lightly spray the basket or use parchment with air fryer holes to prevent sticking.

Air fry: Arrange rolls in a single layer with space between them. Cook 10–14 minutes, flipping at 8 minutes if your air fryer browns unevenly. They’re done when the pastry is deep golden and the sausage reaches 160°F (71°C).

Rest and serve: Let sit 3–5 minutes to set. Serve warm with mustard, ketchup, or a quick honey-mustard dip.