Preheat oven to 350°F (175°C). Spread sourdough bread cubes across two large rimmed baking sheets in a single layer.

Bake for 15 to 20 minutes, tossing halfway through, until completely dry, pale golden, and crisp like croutons. Transfer into a very large mixing bowl.

In a large 12-inch skillet, melt 6 tablespoons of butter over medium heat. Add sliced leeks, diced onion, and celery. Sauté for 8 to 10 minutes until vegetables are soft, fragrant, and lightly caramelized.

Add minced garlic, fresh chopped sage, rosemary, and thyme. Cook for 2 minutes until intensely aromatic. Season with kosher salt and black pepper. Remove from heat.

Pour the warm sautéed aromatic herb mixture over the toasted sourdough cubes in the large bowl. Add chopped fresh parsley and gently toss with a wooden spoon.

In a measuring jug, whisk 2.5 cups warm broth with the beaten eggs. Drizzle evenly over the bread cubes, folding gently until every piece is moistened and absorbs liquid without becoming mushy (add remaining 0.5 cup broth if bread feels dry).

Transfer stuffing into a buttered 9x13-inch baking dish. Drizzle the top with the remaining 2 tablespoons of melted butter.

Cover tightly with foil and bake at 375°F (190°C) for 30 minutes. Remove foil and bake an additional 15 to 20 minutes until the top is deeply golden, shatteringly crisp, and fragrant throughout. Rest 10 minutes before serving!