Preheat oven to 375°F (190°C).
Place the pie crust in a pie dish and set aside.
In a large skillet, cook the onions over medium heat until they are soft and golden brown.
In a mixing bowl, whisk together the cream, milk, eggs, salt, pepper, and nutmeg.
Spread the cooked onions evenly over the pie crust.
Pour the egg mixture over the onions.
Sprinkle the grated Gruyere cheese on top.
Bake in the preheated oven for 35-40 minutes, or until the quiche is set and the top is golden brown.
Let the quiche cool for a few minutes before slicing and serving.