Preheat oven to 375 degrees F.
In a small mixing bowl, whisk together egg and ice water.
Add flour and salt to a food processor. Pulse a few times to combine.
Add the cubed butter to the food processor. Pulse again until it forms a crumb.
Pour in egg/water mixture. Pulse until it forms a thick dough.
Flour a clean surface and a rolling pin. Form the dough into a disc and roll the dough over the floured surface until it's large enough to fit into your pie dish, about ⅓-inch to ½-inch thick (about 11-inch diameter for a 9-inch pie dish).
Carefully transfer the dough to the pie dish. Press it down, then trim the excess dough off the top of the edges. Crimp the edges of the dough using your fingers.
Place the dough in the freezer for 20 to 25 minutes, while you prepare the quiche filling.
In a skillet over medium heat, add the olive oil and onion. Sauté the onion about 5 minutes until tender. Add the sun-dried tomatoes, spinach, and seasonings. Stir to combine and cook another minute to wilt the spinach. Remove from heat.
In a large mixing bowl, whisk together eggs, egg yolk, whole milk, and heavy cream.
Fold in the onion, sun-dried tomato, and spinach mixture. Add feta cheese and stir to combine.
Remove the crust from the freezer. Scoop the filling into the crust. Place in the oven and bake for 45 to 60 minutes, or until the crust is golden brown, the filling is set, and a toothpick inserted in the middle comes out clean.