Heat oil in a large nonstick skillet over medium heat. Add onions and a pinch of salt, and cook until softened, about 5-7 minutes. Add garlic and cook 1 more minute.

Raise the heat to medium-high. Add spinach and another pinch of salt. Cook spinach until very tender but not too mushy. Remove the skillet from heat and set aside. We want the spinach to cool down until it is easy to handle.

In a large-sized bowl, mix together the feta cheese, cream cheese, egg, lemon zest, nutmeg, and dried herbs. Mix until thoroughly combined. Set aside.

Once the spinach has cooled, transfer the spinach into the bowl with cheese. It is best to use tongs. Give it a gentle squeeze to get rid of some of the excess liquid before transferring into the cheese bowl. It's not critical that you squeeze the spinach dry.

Once the all the spinach is transferred. Mix the spinach and cheese together until thoroughly combined. Set aside in the refrigerator. The mixture will be easier to handle if it's chilled.

Preheat the oven to 400°F and adjust the rack to middle position.

Now we're ready to work with the puff pastry dough. Dust your work surface and rolling pin with white flour. Roll out the puff pastry dough into a 10- by 8-inch rectangle. Using a sharp knife, cut the puff pastry into equal quarters or 4 equal rectangles.

With a fork, pierce the dough a few times. Take the bowl of spinach and cheese from the refrigerator. Scoop about a heaping 1/4 cup of the spinach & cheese mixture onto each rectangle of dough and fold over the dough in half. To seal the edges, either crimp the edges with the back of a fork OR fold the edges over and press. The latter method is a better seal, but the former is prettier. You choose! Make a few small slashes through the pastry across the top of the pie.

Line a large baking sheet with a sheet of parchment. Bake the pies on the baking sheet until pastry is well-browned, about 30 minutes.

If you are not baking the pies right away, lay the pies on a parchment-lined baking sheet and place in the freezer for several hours. Once frozen, transfer the pies into a large zip-top food storage bag. Thaw the pies overnight in the refrigerator when you're ready to bake them. Follow step 9.

Serve hand pies warm or at room temperature.