Prepare the crust: Preheat your oven to 375°F (190°C). If using a pre-made crust, gently place it into a 9-inch tart or quiche pan and press it into the edges. Prick the bottom lightly with a fork to prevent bubbling during baking. For a crisper crust, you can bake it blind for 8–10 minutes, then remove the weights and set aside.

Cook the spinach and onions: In a medium skillet, heat olive oil or butter over medium heat. Add chopped onions and sauté until translucent and fragrant, about 4–5 minutes. Add the chopped spinach and cook until wilted and tender, about 3 minutes. Remove from heat and let cool slightly.

Mix the custard: In a medium bowl, whisk together the eggs, heavy cream, salt, pepper, and nutmeg until smooth and slightly frothy. This will create a creamy, flavorful custard for the quiche.

Assemble the quiche: Spread the cooked spinach and onions evenly over the prepared crust. Sprinkle the shredded Gruyère cheese on top, then gently pour the egg and cream mixture over everything. Finally, sprinkle the Parmesan cheese on top for a golden, flavorful finish.

Bake the quiche: Carefully place the quiche in the preheated oven and bake for 35–40 minutes, or until the custard is set and the top is golden brown. To check doneness, insert a knife into the center—it should come out clean.

Cool and serve: Allow the quiche to rest for 5–10 minutes before slicing. This helps the filling set and makes it easier to cut perfect slices. Serve warm or at room temperature with a fresh green salad or light vinaigrette.