Combine water, salt, pepper, and sage in a pot and bring to simmer over medium-high heat. Add chicken and lower the heat to medium-low. Let it slow simmer for about an hour.
Take out the chicken to cool a bit. Pour the broth into a different container and set aside.
Shred chicken using two forks. Cut the long shredded chicken pieces to make it easier to eat.
Add oil to the pot and set the heat to medium.
Slice onion and leek thinly and saute for a few minutes.
Add shredded chicken, mix well and add beans. Mix and saute for a couple of minutes.
Add broth, red pepper flakes, and salt to taste. (Use as much or as little red pepper flakes as you want, based on your own spice tolerance.) Cover and cook for 15-20 minutes.
Stir in heavy cream and add chopped kale.
Cook for a few minutes, until kale is softened.