Thaw pastry sheets. Place the butter in the freezer.
Chop chicken into small pieces.
Peel and grate potatoes. Run potatoes under cold water to remove the starch and squeeze to get all of the water out.
Finely chop the onion. Grate the frozen butter.
In a bowl, combine the chicken, potatoes, onion, butter, mozzarella cheese, seasoning and herbs.
Use one pastry sheet at a time, keep the rest refrigerated. Unroll the pastry sheet. Cut into three strips where the creases are. Take one strip and roll it out a little bit. Cut the strip into three equal parts.
Add the mixture to one side of the pastry. Fold pastry over and pinch the seams together really tight, tuck pinched edges under (so the ends don’t come undone during baking).
Continue with the remaining sheets and chicken potato mixture. (Optional- Make slashes on top of the pies with a sharp knife.)
Beat the egg and water for the egg wash, brush tops of pies.