In the bowl of your slow cooker, combine the onion, carrots, celery, peas, rice, turkey, stock, salt and pepper. Cook on high for 3 to 4 hours or on low for 6 to 8 hours, until rice is tender and soft.
Once rice is done, melt the butter in a medium saucepan with the flour. Cook for a few seconds and then whisk in the milk. Season with a bit of salt and pepper. Cook until thickened and then stir into the soup. Cook for another 20 to 30 minutes until soup has thickened.