Preheat oven to 375°F. Cut bread into cubes. Place on a baking sheet and bake for 15 to 25 minutes until browned and toasted, stirring and checking occasionally.
Chop the leeks (watch the video!): Chop off the dark green stems and the root of the leeks, then slice them in half length-wise. Place each leek half cut-side down on the cutting board, then chop it into thin half-moon shapes. Rinse thoroughly in a colander to remove any dirt. Chop the sage and parsley.
In a skillet, heat the olive oil over medium-high heat. Add celery, onion and leeks with a few pinches kosher salt; cook 6 to 8 minutes, stirring frequently, until tender. Remove from the heat and stir in the herbs.
Place the bread cubes and vegetables in a bowl, add 1 teaspoon kosher salt and a generous amount of fresh ground black pepper, and mix to combine.
Pour the mixture into a 9 x 13 baking dish and add 3 cups vegetable broth. Cover and bake for 20 minutes. Uncover, and bake for about 25 minutes, until browned.