Sweat onions with the oil in a saute pan until translucent, set aside to cool.
Using a food processor, pulse the brisket, onions, BBQ sauce. Do it in batches if it won’t fit. You want to balance chopping it all up and not turning it into a paste. Transfer mix to a bowl for working. This can also be frozen for later use.
Preheat oven to 400F.
Using a round cookie cutter, cut as many circles out of a sheet as you can. The size is up to your preference, I use a 3” cutter. You can reserve the scraps to roll out more puff pastry later. Note: puff pastry will dry out if left uncovered for an extended period.
Place a small spoonful of mix in the center of the circle.
Use a pastry brush or your finger to brush the egg on the outer rim of the circle, and pinch up the three corners. Many videos are available for viewing online on how to do this.
Place finished hamantasch on a parchment-lined baking sheet. Leave about an inch clearance around each hamantasch, as they will puff up a bit in baking.
Brush additional egg wash on the exposed pastry before baking.
Bake 15-20 minutes until pastry is golden brown.
Place on cooling rack. Can be stored in airtight containers in the fridge for up to a week.