Preheat the oven to 300 °F Cut the sourdough into 1/2 inch cubes and place them on a baking sheet. Spray the tops of the cubes with nonstick cooking spray or drizzle with olive oil. Bake for 15-20 minutes to dry out and very lightly toast bread.
To a skillet, melt butter over medioum heat and saute onion, celery and mushrooms until soft and translucent, about 5-7 minutes. Remove from heat and let cool slightly.
To a large mixing bowl, add dried bread cubes, veggie saute, 2 cups of stock (or broth) and 2 whisked eggs. Gently fold to combine and add more broth if needed. You only want to moisten the bread and not soak it to keep the texture toasty.
Transfer the stuffing mixture to a 5-6qt. slow cooker coated with cooking spray. Cover and cook on low for 3-4 hours or until a thermometer reads 160 °F.