Crock Pot Creamy Chicken Stew



2 large boneless chicken breasts, cut into bite sized pieces
1 Tablespoon butter
1 medium onion, cubed
4 small red potatoes, cut into quarters
1 cup baby carrots, chopped in half
¾ cup string beans
½ cup celery, diced
1 (10 ¾ oz.) can cream of chicken soup
½ cup milk
½ cup sour cream
1 (1 oz.) packet dry ranch dressing mix
Salt/pepper to taste
1 teaspoon Parsley flakes