Preheat oven to 400 degrees F. and lightly grease a 9" pie pan with non-stick cooking spray.
Place a piece of parchment paper onto a large cutting board and carefully unfold thawed pastry. Use a rolling pin to smooth out pastry and fit it to a 9" pie pan (I find that one end needs to be elongated by an inch or so).
Transfer to prepared pan and trim pastry to fit pan. Use a fork to poke several holes on the bottom of pastry. Use the same fork or your fingers to crimp pastry edges. Place in refrigerator to keep cold.
Heat oil in a large skillet over medium heat. When oil is hot, add diced shallots and cook, stirring occasionally, until tender and translucent, around 5 minutes. Add garlic and cook for an additional minute.
Add spinach and cook, tossing continuously, until wilted, about 2-3 minutes. Stir in lemon juice, lemon zest, and remove from heat.
In a large bowl, whisk eggs, milk, salt, and pepper. Set aside.
Pull prepared puff pastry from the fridge and transfer spinach mixture into pastry shell. Pour egg mixture on top, then sprinkle with crumbled feta cheese.
Place pie pan on a rimmed baking sheet. Loosely cover the pan with aluminum foil and bake for 20 minutes. Remove the foil and continue baking for another 20 minutes, until the quiche is set (with a slight jiggle in the middle) and the crust is golden brown.
Let the quiche rest for 10 minutes before serving. Garnish with chopped parsley (if using), and serve for breakfast, brunch, or as a main course accompanied with a side salad. Enjoy!