Cube the bread. Cut or tear the sourdough into 1-inch cubes.
Toast the bread. Preheat the oven to 375°F. Put the bread cubes on a rimmed baking sheet or in the dish you are going to bake the stuffing in, and toast for about 15-20 minutes.
Cook the onion, shallot, garlic, and celery. While the bread toasts, melt the butter in a large skillet over medium heat. Once melted, add the onion, shallot, garlic, and celery. Cook, stirring occasionally, until all are softened - about 10 minutes.
Add the herbs. To the cooked vegetables add the fresh sage, thyme and parsley - cook for another 2 minutes to bloom their flavors. Set the mixture aside and let it cool slightly to room temperature.
Whisk the egg and stock. In a large mixing bowl whisk together the egg and chicken stock.
Add the herb mixture and parmesan. To the egg mixture whisk in the herb mixture, parmesan cheese, kosher salt and pepper.
Toss with the bread and transfer to a baking dish. Add the toasted cubed bread to the bowl and toss to combine well. All of the bread should be saturated, with a little additional liquid left at the bottom of the bowl. Transfer to your baking dish.
Bake. Bake in a preheated 375°F covered for 30 minutes and uncovered for an additional 30 minutes until deeply golden brown.