To the basin of a 6-8 quart slow cooker, add the peas and carrots, cream of celery soup, cream of chicken soup, diced onion, minced garlic, mustard powder, salt, pepper, chicken broth, and brown rice. Stir until well mixed and smooth. Add the chicken pieces to the mixture and stir to evenly distribute. Cover and cook on HIGH for 4 hours or until the rice is thoroughly cooked but not mushy. Turn the slow cooker off and stir in the shredded cheese, replace the cover and allow the cheese to melt. Approximately 15 minutes. Top with parsley and serve!