In a stand mixer fitted with a paddle attachment, mix together the flour and salt until combined. Add the butter and mix on medium speed until you achieve hazelnut size pieces of butter in the mixture. Add just enough ice cold water to bring the mixture together when presses. Wrap with plastic wrap and refrigerate for for 45 minutes.

Heat a pan over medium high heat. Once hot, add the oil and sauce the onions for 5 minutes. Add the diced mushrooms and cook on high heat until cooked, 5 minutes. Season with salt and pepper. Set aside to let it cool. Once cool, mix it together with the diced pork and set aside.

Preheat your oven to 375F. On a lightly floured surface, roll out your dough until it's 1/8" thick. Cut out 3" squares. Place a spoonful of filling in the center and brush the edge with a bit of egg wash. Fold the other half of the square over to make a triangle. Repeat.

Place the puffs on a baking sheet and brush with egg. If the puffs seem to be getting very soft, refrigerate for 15 minutes. If not, bake for 20 minutes or until golden brown.