Preheat the oven to 400°F. Line two baking sheets with parchment paper.
Remove the mushroom stems and wipe any debris with a damp paper towel or cloth. Do not wash. Mushrooms act like a sponge when washed.
Combine the cheese, garlic, salt and sundried tomatoes. Add enough of the marinara to coat ingredients. Adding more than you need will result in soggy empanadas.
Unfold the puff pastry and roll it out to smooth out any wrinkles. Stamp out 4" circles with a biscuit, cookie cutter or a drinking glass.
Whip the egg with a teaspoon of water.
Working with one empanada at a time, brush the perimeter of the puff pastry circle with the egg wash. Spoon or scoop one tablespoon of filling into the center of each circle. Fold in half and gently press edges together. Using a fork, crimp the edges closed. place on the baking sheet.
Repeat with each empanada. Brush the puff pastry with egg wash and bake for 20 minutes or until browned.