01 -
Preheat oven to 350°F (175°C).
02 -
Slice leeks into ½-inch rounds. Soak them in lukewarm water to remove dirt, then strain and pat dry.
03 -
Heat a skillet on medium-high. Add leeks and salt, cook for 5 minutes. Lower heat, stir in butter and olive oil, cover, and cook for 20-30 minutes, stirring occasionally until soft and caramelized.
04 -
Cut bread into 1-inch cubes. Spread on a sheet pan and toast for 20 minutes until golden and dry. Transfer to a large mixing bowl.
05 -
In a separate bowl, whisk together eggs, milk, half and half, Dijon mustard, salt, pepper, chives, thyme, and nutmeg.
06 -
Grease a 9x13-inch baking dish. Layer the bottom with ½ cup cheese. Add half of the leek and bread mixture, top with ½ cup cheese. Add the remaining leek mixture and pour in the custard, pressing bread cubes to soak. Let sit for 10 minutes. Top with remaining custard and cheese.
07 -
Bake for 1½ hours, until puffed and lightly browned.