1. Preheat oven to 350ºF. Spread bread cubes in a single layer on a large baking sheet. Bake until dry and golden, 15 to 20 minutes. Cool.
2. In a large pan over medium heat, bring 4 cups of broth to a boil. Add 1 tsp. each salt and pepper and wild rice, stir well, cover, reduce heat and simmer for 60 minutes. Drain off any excess liquid.
3. In a skillet, melt butter over medium heat. Cook onion and celery, stirring often, until tender, about 8 minutes. Raise heat to medium-high, add mushrooms and cook, stirring often, until liquid evaporates, about 6 minutes. Stir in herbs, 1 tsp. salt and 1/4 tsp. pepper.
4. Mist inside of slow cooker with cooking spray. Add wild rice, mushroom mixture, bread and remaining 1 1/2 cups chicken broth; stir well. Cover and cook on high for 2 hours.