Rinse the rice under running water. Place the uncooked rice, chicken breast, onions celery, carrots, garlic, bay leaves, chicken broth, water and seasonings in a slow cooker. Cover and cook on high for 3-4 hours or on low for 7-8.
In the last 1/2 hour of cooking, remove the chicken from the slow cooker. Allow to cool slightly before shredding using two forks.
When the rice is done cooking, add the shredded chicken back into the slow cooker. Melt the oil in a saucepan. Add the flour and let the mixture cook for 1 minute. Whisk the mixture slowly while adding milk. Continue to whisk until all lumps have dissolved. Allow the mixture to thicken and become creamy.
Add this creamy mixture to the slow cooker. Stir to combine. Add more water or milk until you reach desired consistency and add salt and pepper to taste.