Preheat your oven to 375°F (190°C). Place the pie crust into a 9-inch pie dish and crimp the edges. Prick the bottom with a fork in a few places.

Blind-bake the crust by lining it with parchment and filling with pie weights or dried beans. Bake for 12 minutes, then remove weights and bake 5 more minutes until light golden.

Heat olive oil in a skillet over medium heat. Add the chopped onion and a pinch of salt, cooking 4-5 minutes until translucent. Add garlic and cook another 30-60 seconds until fragrant.

Add fresh spinach to the skillet in batches, stirring until wilted. If using frozen spinach, squeeze out excess moisture before adding. Transfer mixture to a colander to cool and press gently to remove excess liquid.

In a medium bowl, whisk together ricotta, eggs, milk, Parmesan, nutmeg, salt, pepper, and red pepper flakes if using. Once smooth, fold in the cooled spinach and onion mixture.

Pour the spinach-ricotta mixture into the prepared pie crust, smoothing the top. Scatter shredded mozzarella or Gruyère on top if using.

Place the quiche on a baking sheet and bake for 35-45 minutes, until the center is set and the top is lightly golden.

Let the quiche rest at least 10-15 minutes before slicing.