Spray 6-quart slow cooker with cooking spray. Pour onion mixture into slow cooker. Stir in thyme, bay leaves and carrots. Top with chicken. Cover; cook on Low heat setting 6 to 8 hours. Stir in mushrooms. Cover; cook 20 to 30 minutes longer. Remove chicken and vegetables from slow cooker. Discard bay leaves. Increase heat setting to High. Mix cornstarch and water; add to slow cooker, stirring with whisk. Cook and stir 2 minutes, until thickened. Stir in parsley and lemon juice. Serve chicken, vegetables and sauce over mashed potatoes, if desired.