Heat the oven to 350 degrees F.
Roll out the dough on a floured surface until you have a dough sheet about 1/8 inch thick. You can use a Pasta maker for this, it works great.
Cut out dough circles 5 inches in diameter. You can purchase 5 inch cookie cutters, these work very well.
Flour the dough circles and place them on a plate. Then cover with a lightly moist towel to keep them from drying out.
Place a dough circle on a floured surface. Using a brush, brush the egg wash around the dough circle.
Add about 2 TBSP of the onion filling and fold over the circle making a half moon shape. Seal well.
Using a fork, push down on the dough edge, both further sealing and making a design.
Place the folded Empanadas on a parchment covered cookie sheet.
Brush all the Empanadas with the egg wash and, if desired, lightly sprinkle with the salt.
Bake for 25-30 minutes until the Empanadas are just beginning to lightly brown.
Place on a rack to cool slightly before serving.