Preheat oven to 325° and line a large baking sheet with parchment paper.
Cube the sourdough bread, then place the cubes on the prepared baking sheet in an even layer. Bake in the preheated oven for 30 minutes.
Meanwhile, brown the sausage (if using) in a skillet over medium heat, crumbling it as it cooks. Drain and set aside.
Melt the butter over low heat in a medium saucepan. Stir in the rosemary, thyme, salt, chicken seasoning, parsley, pepper, and garlic powder until well combined.
Add the celery and onion to the herb butter mixture and stir to combine. Bring the butter to a simmer, and simmer for 10 minutes.
Once the bread cubes are done toasting in the oven, transfer them to a 9x13 baking pan. Add the Italian sausage crumbles and pour in the butter mixture as well. Toss to combine. Move to step 7 OR to make ahead, cover and store in the fridge for up to 24 hours. When ready to bake, move to step 7.
Preheat the oven to 425°. Stir the chicken broth in with the stuffing, tossing to combine. Cover with aluminum foil and bake in the preheated oven for 20 minutes. After 20 minutes, uncover and continue to bake for another 15 minutes. Serve!