Clean the leeks, slice them, and sauté in a dry skillet over medium heat for about 5 minutes. Add salt and pepper, then incorporate butter and cover to cook for another 20-30 minutes until tender.

Preheat your oven to 350°F (175°C). Cut the bread into one-inch cubes, spread on a baking sheet, and toast for about 20 minutes until golden and lightly crisped.

In a mixing bowl, whisk together eggs, milk, half-and-half, Dijon mustard, and seasonings until well combined and smooth.

Grease a 9x13-inch baking dish and layer half of the Gruyère cheese, followed by the toasted bread cubes, and sautéed leeks. Repeat with remaining cheese and bread.

Pour the custard mixture over the layers and allow to rest for 10 minutes. Bake uncovered for approximately 1.5 hours until puffed and golden on top.