As you allow the mushroom mixture to cool, you can start your mini pie crusts.
First, prepare two regular-sized non-stick muffin tins by giving each cavity a spritz of non-stick cooking spray. While completely optional and a little time consuming, you can ensure easy removal of your mini picnic pies by cutting out strips of parchment paper about ½ an inch wide and 6 inches long, and using them to line each cavity so that the ends stick up on either side. Once done, the parchment ends can be used as handles to lift out the pies.
Working with one ball at a time on a large, lightly floured service, roll out the dough into an oblong shape, 1/8th – 1/4th inch thick. Using a 6-inch round cookie cutter, cut rounds from the dough and set them into the muffin tin so that they line each cavity. Once the first muffin tin is full, transfer the whole thing back to the fridge to say cold, and repeat the process to fill the second muffin tin with rounds of pie crust.
Using a slotted spoon to avoid transferring too much liquid to the pies (which could result in a soggy bottom crust), fill each muffin cup with the cooled mushroom-gruyere filling. Crimp the top edges of the crust inward so that it slightly folds over the tops, leaving the very center open so that you can see the filling. Brush the edges of the dough with the egg wash.
Bake in the oven for 30-40 minutes, until done. Let them rest 5 minutes, and then carefully lift the picnic pies from the muffin tins. You can serve these immediately or at room temperature. If making these ahead to store, follow the instructions below in the Recipe Notes.