Preheat oven to 350 degrees F. Convection oven setting is ideal.
Heat oil in a medium-sized skillet. Add onions and ¼ teaspoon of Kosher salt and cook over medium-low heat with a lid on for 8-10 minutes, stirring often, until very soft and starting to caramelize.
Add the sherry and let cook for 5 minutes on low heat. Set aside to cool.
Butter or oil the inside of the 11” pie plate.
In a large bowl, whisk eggs and milk. Whisk in onion powder, ½ teaspoon Kosher salt, and flour, and mix until smooth.
Stir in the cheese and chopped chives.
Pour the egg mixture into the prepared dish.
Using tongs, gently place the slightly cooled caramelized onions into the egg mixture, distributing them evenly.
Bake for 40-45minutes or until quiche is set.
Allow to cool for 5 minutes before slicing and serving.