Cut pancetta into 1/2 inch chunks.
Heat skillet on high heat and cook pancetta until only crispy in parts, then allow to drain on a paper towel lined plate.
Add onions to skillet and saute until scent releases then add Brussels sprouts, Saba and vinegar.
Saute until sprouts become a little wilted then add bacon.
Toss to combine and set aside.
Sprinkle flour on rolling pin and on a clean, dry, flat surface.
Remove thawed pastry sheets from the fridge gently roll out just a little, keep the shape of the pastry sheet.
With a cookie cutter, cut out as many circles as you can.
Collect the trimmings of the pastry and stack them, then gently roll them to create the same thickness as the original pastry sheet, then cut out as many circles as you can. Discard the excess trimmings.
Line baking sheets with parchment paper, then place your pastry circles on top, leave a little space in between each pastry circle.
Whisk together the egg and water in a bowl, then brush each pastry circle with the egg wash.
Place a handful of cheese on each pastry circle then top with the Brussels sprouts mixture.
Place in a preheated oven at 400 and cook until pastry is golden (approx. 10 minutes)
Allow to cool a little before serving.