This is not your grandmother's casserole. For one, it's devoid of any canned soup or boxed mixes and yet, even without those timesavers of decades past, it still comes together in a snap. The slow cooker brings this all-American comfort classic to the table without any trouble at all.
Want to make it vegetarian? Double the amount of mushrooms, omit the chicken, and switch to vegetable broth. Hate mushrooms? Just leave them out and keep everyone happy.
If your slow cooker has a brown or sauté function, melt the butter in the slow cooker and add the shallot and celery. Cook until they are soft and translucent, then switch the slow cooker function to high heat. Add the rice, mushrooms, parsley, salt, garlic powder, and paprika, then pour in the milk and chicken broth.
If your slow cooker does not have a brown or sauté function, turn the slow cooker on to high heat and melt the butter. Add the shallot, celery, rice, mushrooms, parsley, salt, garlic powder, and paprika, then pour in the milk and chicken broth.
Stir the ingredients together, then add the chicken.
Cover the slow cooker and cook for 4 hours on high or 7 hours on low. Taste and add more salt if needed, and serve.