Creating Savory Leek and Gruyere Bread Pudding is simple when you follow these easy steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or cooking spray.
2. Prepare the Bread: Cut the day-old bread into cubes and place them in a large bowl. Set aside to allow them to dry out slightly.
3. Cook the Leeks: In a large skillet, melt the butter over medium heat. Add the sliced leeks and sauté until they are soft and translucent, about 8-10 minutes. Remove from heat and let cool slightly.
4. Make the Custard Mixture: In a separate bowl, whisk together the heavy cream, milk, eggs, Dijon mustard, salt, and black pepper until well combined.
5. Combine Ingredients: Add the sautéed leeks to the bread cubes. Pour the custard mixture over the bread and leeks, gently stirring until the bread absorbs the liquid.
6. Add Cheese: Fold in the grated Gruyere and Parmesan cheeses, reserving a small amount for topping.
7. Transfer to Baking Dish: Pour the mixture into the prepared baking dish. Spread it evenly and sprinkle the reserved cheese on top.
8. Bake: Place in the preheated oven and bake for 40-45 minutes or until the top is golden brown and the custard has set.
9. Cool: Remove from the oven and allow it to cool for about 10-15 minutes before serving. This helps the pudding firm up for easier slicing.
10. Garnish: If desired, sprinkle fresh herbs on top for added flavor and a pop of color before serving.

These steps will guide you in creating this incredible bread pudding effortlessly.