Cube and dry bread overnight. The night before, cut the sourdough bread into cubes and place them on a large baking sheet in a single layer, letting them dry out.
Prep for baking. Preheat the oven to 400℉. Spray a 9x13 casserole dish with a layer of baking spray and set aside.
Sautee veggies. Place a large saucepan over medium heat and drizzle the olive oil. Let the olive oil warm slightly. Add the onion, carrots, and celery to the saucepan and saute until the onions become slightly translucent, about 5 minutes.
Add garlic and herbs. Next, add the garlic and season with salt and pepper. Add the fresh parsley, rosemary, and sage, stirring to combine. Let the mixture cook for 1 minute.
Add broth. Add the chicken broth and stir to combine.
Mix in bread cubes. Take the pot off the heat and add the cubed, dried bread. Using a wooden spoon or spatula, stir to combine, making sure everything is well incorporated.
Assemble in a casserole dish. Transfer the mixture to the prepared casserole dish. Top with the cubed butter.
Bake. Bake at 400℉ for 30 minutes.
Cool and serve. Remove the stuffing from the oven and let it cool for 5 minutes before serving.