Place the pie crust pouches to stand at room temperature about 15 to 20 minutes.
Preheat the oven to 375 degrees. Spray 48 miniature muffin cups with non-stick cooking spray.
Open and unfold one pie crust, and press out any fold lines. With a 2 and 1/2 inch floured biscuit cutter, cut 12 rounds. Repeat with remaining pie crusts. In the prepared muffin cups, press 1 round of pie crust into the bottom and up the sides of the cup.
In a 4-cup measuring cup, combine the eggs, milk, and the salt. Whisk well. Set aside.
Place one tablespoon of cheese into each of the muffin cups. Evenly divide the chopped mushroom among the cups, about 1/2 teaspoon.
Pour egg mixture into each of the 48 cups, filling to within 1/4 inch of the top. Bake at 375 degrees until golden brown, about 25 to 30 minutes. Cool slightly and serve warm.