In a frying pan, melt the butter over medium heat. Add the sliced onions and sauté until they are caramelized, about 15 minutes.

Season with salt, pepper, and nutmeg (if using). Remove from heat and let cool slightly.

In a mixing bowl, whisk together the eggs and heavy cream until well combined.

Stir in the grated cheese and the cooled caramelized onions.

Preheat the oven to 180°C (350°F).

Place the pre-made pie crust in a pie dish. Pour the onion and egg mixture into the crust, spreading it evenly.

Bake in the preheated oven for 30-35 minutes, or until the filling is set and the top is golden brown.

Let the pie cool for a few minutes before slicing and serving.