Preheat the oven to 350 F (180 C).

For the custard

1.  Whiz all the ingredients just until mixed in a blender, about 5 seconds.  If you don’t have a blender, just whisk everything together.

For the filling

1. Set a skillet over medium heat.  Drop in the onion and a little bit of bacon fat if you saved it from cooking the bacon.  If not, use about 1 teaspoon of olive oil.  Stir the onions until they’re translucent, and then add the spinach. 
2. Stir to cover the spinach with the fat and onions, and then cover the pan to let the spinach wilt.  After about 2 minutes the spinach should be tender.  Pour the contents of the pan onto a cutting board.  Chop the spinach finely.  Scoop the spinach into a fine-mesh strainer and press out any excess water from the spinach.
3. Toss together the spinach and crumbled bacon.  The bacon helps break up the spinach so that it doesn’t get clumpy in the pie.

For the quiche

1.  Sprinkle a piece of parchment with a little flour.  Sprinkle some more flour on your pastry, and then place it on the parchment.  Roll out the dough into a sheet about 1/8″ thick that’s big enough to fit into a glass pie plate.  Check the size by placing the pie plate on top of the pastry then inverting the parchment into the pan.  If the dough extends just above the rim of the pie plate, you’re good to go.  If it’s not quite there, flip everything out and roll a little bit more. 
2. Once you’ve rolled the dough to the right size, use the parchment to transfer the dough to the pie plate.  Press the dough into the plate, and then trim away the excess dough with a knife around the inside edge of the plate.  Save the scraps for decorating the pie later.

3. Pour a little bit of the custard into the bottom of the crust.  Scatter the spinach/bacon mix and potatoes or ham over that.  Crumble the feta over the top, and then pour the custard to cover everything.

4. Mix the egg yolk with about 1 teaspoon of water with a fork.  Use a pizza wheel to cut the scraps of pastry into thin strips.  Take 2 strips of dough and twist them together, using the egg wash to help everything stick together.  Paint a little egg wash around the inside edge of the pie.  Place the twisted pieces of dough around the top of the pie.  Brush the edge of the pie with egg wash.

5.  Bake for 40-45 minutes until the pastry is lightly brown and crisp and the pie is no longer jiggly in the center.