Prepare Leeks: Trim the leeks, using only the white and light green parts. Clean thoroughly under running water. In a skillet, melt butter over medium heat, add leeks, and sauté for 25-30 minutes until tender.

Toast Bread: Preheat oven to 350°F (175°C). Cut bread into cubes, spread on baking sheet, and toast for about 20 minutes until golden.

Make Custard: In a mixing bowl, whisk eggs, then add milk, half-and-half, Dijon mustard, salt, white pepper, and nutmeg until smooth.

Layer Ingredients: Grease a baking dish. Layer half of the bread, leeks, and Gruyère cheese, then repeat, pressing down lightly.

Soak: Pour custard evenly over mixture, ensuring bread is soaked. Let rest for about 10 minutes.

Bake: Bake in the oven at 350°F (175°C) for approximately 1.5 hours until puffed and golden brown.