Spray 5-quart slow cooker with cooking spray. Mix onions, broth, wild rice, carrots, mushrooms and 1 teaspoon of the salt in slow cooker. In large bowl, mix melted butter, remaining 1 teaspoon salt and the pepper. Add chicken; toss to coat. Place chicken on top of rice mixture in slow cooker. Pour any remaining butter mixture over chicken. Cover; cook on Low heat setting 6 to 7 hours or until liquid is absorbed in rice.