Preheat the oven to 325 degrees F. Spread out the cubed bread onto a large baking tray. Bake until golden in spots, about 30 minutes, tossing once halfway through.
In a large, deep skillet, add the butter and melt over medium heat. Once melted, add the celery, onion, and carrot. Cook until the vegetables are softened and starting to caramelize in spots, about 7 to 8 minutes, stirring occasionally. Add the garlic and cook 30 seconds, stirring constantly.
Add the celery mixture to the bowl with the toasted bread cubes along with the parsley, thyme, rosemary, sage, salt, poultry seasoning, and black pepper. Toss gently to combine. Add the egg and gently toss again to coat.
Add the broth 1 cup at a time, stirring in between additions. You might need up to 5 cups of broth, depending on how dry the bread is. You want the bread to be moistened, but not fully saturated.
Spray the inside of an 8-quart capacity slow cooker with cooking spray. Transfer the stuffing mixture to the prepared slow cooker. Cover and cook on LOW heat for 4 hours. Don’t open the slow cooker during this time. Serve warm.
Notes
If desired, you can use store-bought dry bread cubes. However, for the best flavor, it’s best to use fresh bread and dry it out in the oven.