Preheat the oven to 375°F (190°C). Grease a 9-inch pie pan to prevent sticking.

In a large skillet over medium heat, add 2 tablespoons of oil. Once hot, add the chopped onions and cook until translucent, about 8 minutes.

Add the minced garlic and cook for another 1-2 minutes until fragrant.

Gradually add the spinach in batches, allowing it to wilt. Add 1/2 teaspoon of kosher salt and 1/4 teaspoon of crushed red pepper flakes.

Once the spinach has wilted, set the cooked veggies aside to cool slightly.

In a large bowl, whisk together the eggs, heavy cream, and the remaining 1/2 teaspoon of salt.

Stir in the shredded mozzarella and the cooled veggie mixture. Mix until well combined.

Pour the quiche mixture into the prepared pie pan.

Place the quiche in the oven and bake for 20-25 minutes, or until the edges are lightly browned and the center is set.

Allow the quiche to cool for 10 minutes before slicing.

Serve warm or at room temperature.