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 allowing them to dry out.
Prep for baking. Preheat the oven to 400 degrees F. 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. Allow the olive oil to warm slightly. Add the onion, carrots, and celery to the saucepan and allow the vegetable mixture to saute until the onions become slightly translucent about 5 minutes.
Add garlic and herbs. Next, add the garlic and season the vegetable mixture with salt and pepper. Add the fresh parsley, rosemary, and sage to the vegetable mixture, stirring to combine. Allow the vegetable mixture to sweat 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 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. Transfer to the preheated oven and bake at 400 degrees F for 30 minutes.
Cool and serve. Remove the stuffing from the oven and allow it to cool for 5 minutes before serving.