01 - Preheat oven to 375°F.
02 - Roll out pastry and line a 9-inch tart tin. Trim edges and prick base with a fork. Chill for 10 minutes.
03 - Line pastry with baking paper, fill with baking beans, and bake for 15 minutes. Remove paper and beans, then bake for another 5 minutes until golden.
04 - Cook bacon lardons in a skillet over medium heat until crisp. Drain on paper towels. In the same pan, sauté onions until soft, about 3 minutes.
05 - In a large bowl, whisk eggs, heavy cream, milk, salt, pepper, and nutmeg until combined.
06 - Spread cooked bacon, sautéed onions, and half the grated cheese evenly over the pastry base. Pour in egg mixture. Top with remaining cheese and sprinkle with chives if using.
07 - Bake for 25 to 30 minutes until filling is set and golden.
08 - Allow to cool for 10 minutes before slicing and serving.