Preheat the oven to 350 degrees Fahrenheit and spray a pie pan or a removable bottom tart pan with non-stick spray.
Place the pie crust dough on the pan and cut the excess dough from the around the edges if using a tart pan. If you are using a pie pan, pinch the edges around the edges of the pan.
Poke holes in the bottom of the crust using a fork then place some parchment paper in the middle along with some pie weights or uncooked regular beans and bake for 10 minutes.
Remove the pan from the oven and remove the parchment paper and beans and bake for another 5 minutes, or until crisp, but still pale in the center.
Remove the pan from the oven and set aside to cool for 10 minutes.
In the meantime, heat the butter in a medium pan over medium heat and add the onion. Cook until the onion becomes translucent, then add the garlic, salt, pepper and thyme and cook for an additional minute or so, or until the garlic becomes fragrant.
Add the spinach and cook for 2-3 minutes, or until the spinach has reduced significantly. (You may need to add the spinach in batches if your pan is really small.) Remove the pan from the heat and drain the excess liquid from the spinach.
Spoon the spinach mix onto the pie crust in an even layer then add the feta in an even layer on top.
In a medium size bowl, whisk the eggs and heavy cream together then pour it over the spinach and feta mix.
Bake for 35 to 45 minutes, or until the edges have puffed up and the top has turned a light golden color.
Remove the pan from the oven and let the quiche cool for 10 minutes before slicing and serving.