Preheat your oven to 375°F (190°C). If using store-bought pie crust, let it come to room temperature.
Heat olive oil in a large skillet over medium heat. Add onions and cook until translucent, about 5 minutes.
Add garlic and cook for another minute until fragrant.
Add spinach in batches, stirring until just wilted. Remove from heat and let cool slightly.
In a large bowl, whisk together eggs, heavy cream, nutmeg, salt, and pepper.
Drain any excess liquid from the spinach mixture to prevent a soggy quiche.
Line your pie dish with the crust, crimping the edges.
Spread the spinach mixture evenly across the bottom of the crust.
Sprinkle crumbled feta cheese over the spinach.
Pour the egg mixture over everything.
Bake for 45-50 minutes, or until the center is set and the top is lightly golden.
Let rest for 10-15 minutes before serving.