Prepping bread cubes: Cut bread into 1-inch cubes then add to a rimmed baking sheet in a (mostly) single layer.
Bake cubes 30 minutes at 350 degrees or until golden brown and lightly crisp. Give them a good shake about halfway through bake time. Note: day-old bread toasts faster. Cool completely then transfer to a large bowl.
Spray the inside of your crockpot including the bottom and sides with cooking spray (can brush with olive oil instead). Add dry bread cubes, top with pats of butter then season with salt and pepper.
Next top with chopped parsley, thyme and sage. Add uncooked onions and celery then crumble raw sausage over the top. Pour in turkey (or chicken) stock.
Cover the top of the crockpot with a dishtowel then place lid over the towel. This will keep the condensation off the stuffing so it's crispy outside, moist inside.
Turn crockpot to high for 4 hours. After 30 minutes, give the stuffing a stir, breaking up any larger pieces of sausage and blending it into the bread cubes.
Continue slow cooking with the dish towel and lid in place. You know your own slow cooker, so check the stuffing after 3 hours cook time. It's done when the vegetables are tender, sausage is cooked through and and you have crispy sourdough bites with moist stuffing inside.